I believe the corrupt comes from human nature in that politicians are given too much power and prestige which they then begin using to their advantage. Some people are already weak morally and when you add power and pressure to the mix then what's on the inside shows up.
We have created so much red tape and so many laws that the politicians feel they must cheat to win and cheat to get their issues passed. I don't feel they all get into politics looking for this it just happens. If you are not a strong person then corrupt people come around and persuade you to break a little rule here and break a little rule there and before they know it they are so ashamed of the game they are playing they just give up and do what the rest do to win cheat.
Our government is way too big, it is way bigger then it was ever intended to be and there is not enough checks and balances to keep people honest. We pay them too much, we have allowed them to create laws that give them retirement for the rest of their lives regardless of how many years they are in office how many places can you work where that happens....?
They take advantage because we allow them to take advantage. They create laws that actually go against the constitution and we the people sit back and do nothing because we have lost the power to do it. Our vote no longer counts it is the ones we elected that make the votes and we still just sit back and allow this to happen.
Our country has became the same as England was when we left it to come over to America. We have forgotten the real reason why the founders of our country and constitution wrote the constitution the way they did. If we were truely following the constitution then we wouldn't have tax on this and tax on that and a government so big we can't pay for it or regulate it....