Short answer: it's easy to complain about a problem, it's much harder to actually do something to fix it.
Long answer: let's say government has decided to raise your income tax and that makes you angry, so you complain about it. That requires hardly any effort, in fact many people seem to like complaining, so it's actually an enjoyable activity rather than a chore. This probably explains most of the cases when so many people seem to be unhappy and complain about government that one might think that we are on a verge of a revolution, but nothing really seems to actually happen.
But let's say you want to actually do something to fix the problem and are willing to spend some effort to do it.
First thing you need to do is to figure out a better alternative to the government decision. If they have decided to raise income tax to cover deficit, then what would be your solution to the problem? Do nothing and let the deficit grow? Raise some other tax instead? Or cut government spending? If so, which part? Defense? Health care? Many protest movements have this problem: they see a problem and are willing to spend effort to do something about it, but fail to come up with a realistic plan how they would actually fix it. Or they come up with dozens of plans, each incompatible with the others and fall to infighting.
But you are sure a better solution exists and are willing to do some work to find it. If you are lucky, you may find someone who already has come up with a better solution that you agree with, then you only need to back them: vote for them in the elections, promote their solution in blogs and so on, you are good to go.
But let's say that you come up with a new solution that no-one else is promoting. Let's even say that your solution is objectively better in every way and you are able to convince most people you discuss with that it is indeed a good idea.
In an ideal democracy, you would present your solution to public, convince majority of them that this is indeed an optimal solution and then it would become the government's policy. If you are lucky enough to live in a place that allows citizen initiatives or other forms of direct democracy, then you might be able to got this route.
The rest of us need to deal with representative democracy. You might be able to convince some existing candidate of your solution. But even if you find a candidate that agrees with your solution, she may have opinions about other issues that you do not agree with and therefore can't vote for her. Or perhaps she as a person is alright, but belongs to a party that has policies she must follow and you disagree with.
So you have gone through other options and the only way to get your solution implemented is to stand in elections yourself and are willing to put in the significant amount of work it requires and deal with the fact that despite all the work you are very unlikely to be actually elected. If you are lucky, you can find an existing party that has policies you agree with and are willing to accept you as their candidate and give you freedom to promote you solution. But likely your country has only a limited number of parties and each one has policies that you disagree with. Then you either need to compromise on some issues, agree to vote along party lines despite disagreeing with them and hope to eventually change them from the inside, or establish a new party or run as independent, which allows you to keep your integrity but requires significantly more work and reduces your chances to be elected basically to zero.
But let's say you put in the work, collected the signatures, kissed the babies and somehow manage to end up as a candidate in elections. You even get to keep a press conference about your candidacy and, most importantly, your solution. Then comes the Q&A session:
-"Now, are there any questions?"
-"Sure, your solution to sounds pretty swell, but what's your stand on gay marriage? Immigration? Abortion? Foreign policy? Dozens of other budget issues?"
Every time you don't give a firm opinion, you are accused of being vague or incompetent, that voters have the right to know what button you will push if the issue comes up to voting, or are you simply going put in empty vote every time? And each time you do give a firm answer, either your own opinion on whatever your party line is, some part of the voters are going to go: "Meh, I really liked his solution, but I can't really vote for someone who thinks that way about issue X".
So in the end, even if the majority of the voters would agree that your solution is the best, the only votes you will get are from people who happen to agree with you also on all the other secondary issues, no matter if you have really thought about them or are just following the party line. So no matter how superior your solution is, your probability to get elected depends mostly on the completely different issues.
But let's say you put in even more work, the money for advertising, kissed huge amount of babies and somehow managed to give sufficiently reasonable answers to all the other issues that against all odds you find yourself at a position of power, a freshly elected member of your nation's law-making body. Now you are finally in a position to implement your solution to national policy.
Unless, of course, your party ends up in opposition. No matter how good your solution is and even if most other representatives agreed with it, generally government does not implement any ideas from opposition, no matter how good they might be. Or even if your party does end up in government, but your party leadership refuses to back your idea, perhaps because it would harm one of their major campaign funders. Or if you have coalition government, even if your own party would back the idea, but any of the other government parties makes it their condition that government will not back this solution, perhaps because it would hurt their contributors.
So the government does not take your solution into their agenda, but you could still go rogue and put your proposal to vote independently. Opposition representatives are likely to back it, but government party representatives are going to vote against it, event if they would personally agree with it. And since government parties are the majority, it is not going to pass, but at least you had your shot. But now you and any other representative who voted against their party line are going to find yourself blacklisted: all your future suggestions will be shot down on principle, any party co-operation or campaign support is going to go away and you'll find it hard to make it to candidate on any future elections. Even the opposition parties won't take you, after all, you are might go rogue also on them if they end up in the government.
So perhaps you do the wise thing and accept that your solution is not going to happen this term, but perhaps things will be different after the next elections. After all, now you have the experience, support base and publicity, so you have a pretty good chance to get re-elected and perhaps then your proposal will end up on government agenda. So you bide your time, all the time promoting your idea to you fellow representatives and party leaderships, so that your chances are better next time.
So the next election time comes and you start your campaign trail, more experienced and confident. And face almost empty halls, with a few former voters expressing their opinions:
-"What the hell man? I really believed in you and your solution, I voted for you and told all my friends to vote, too, but what happened? You just sat on your ass for four years doing nothing, and now you have the nerve to ask for my vote again? Forget it. I should have known that all politicians are liars. I'm through with you and voting, except this one last time I'm going to vote for whoever is running against you."
So having lost your main supporter base you fail to get re-elected and become disillusioned and bitter. Perhaps you quit politics altogether, or perhaps you just become more pragmatic: forget your own opinions and ideas, just support whatever cause you think will get you most votes or campaign funding, maybe try to land a lucrative post-term job by supporting whatever laws one of the big corporations wants to see implemented.
So that, my friends, is the reason why most people complain instead of standing in elections themselves. Unless they want to become career politicians, in which case it's better to forget about the actual problem, go straight to the disillusionment part and take it from there.
(Addendum: That may all sound a bit pessimistic, so let me end with some hope. End of segregation, equal rights of women and gays, environmental issues, all have started with someone realizing: "Hey, this is wrong, we really should fix this" and then convincing others that yes, perhaps things would indeed be better if we change this thing. It is often long and uphill battle, but despite all the flaws in representative democracy, once the majority of people feels that a fix is necessary and consider it important enough to vote accordingly, eventually the change will happen.)
The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man." -