Not necessarily 'bought off' as in handing over cash for favor, but a lot of politicians are already part of 'the machine' and profit handsomely from it, so their personal goals line up.
And that's the reason term limits are needed for each and every political office. Of course nothing on the order of a constitutional amendment is even remotely possible in the current US political system (that's likely to persist for a long long time to come).
Term limits are not the solution! In fact it will just exacerbate the problem. If Congress ends up as perpetual freshmen, who do you think will end up with an even higher proportion of power in Washington? The lobbyists and the staffers. These people have the insider knowledge, the connections, and the influence and they will be able to steamroll over an endless parade of newbie lawmakers.
As someone said higher up, the reason nothing is ever done is because the congressmen's incentives align with the banks and the power brokers. The only solution is to ensure that those with voting power in Washington have incentives that are the opposite of the banks. Term limits do not do this.
Possible solutions: Pay Congress and important oversight positions a salary proportional to their importance--somewhere in the multiple millions per year. Make lobbying any government official illegal (anything beyond writing a letter to your Congressmen). A lifetime ban from working for any company that is affected by decisions made while in office. 100% publicly financed campaigns.
I don't think term limits fix what you're asserting. If anything, they just codify when people get out of office and go into particularly posh jobs with the people who were lobbying them.