Every politician is completely sincere.

This is patently, staggeringly false. Politics isn't Wikimedia. There are completely different motivations, rewards and therefore different people get into it. There is money, power, status involved and it's a competitive setting.

What do you think politicians think of themselves when they deceive, become corrupt, sacrifice the interest of the state for that of their own or their party, or go into the pocket of another nation/businessman/organised crime? Calling them "sincere" and acting for the good of the world sounds naive, like you don't believe in the existence of cynical people. As long as the interest of someone else than the voter is being served, and the voter is being deceived about that, that's not sincerity.

EDIT: the amount of upvotes the parent comment got is a bit scary. Either I'm missing something important, or there's a lot of very innocent readers around...