And indeed they are, and this is the heart of the problem. They're just regular people like us, subject to the same prejudices, temptations, motivations, etc. We shouldn't be surprised when the motivations that are built into the system actually do something.
More concretely, we've vested incredible power with our politicians. That makes them huge targets for those with vested interests (whether those be corporation, special interests, foreign governments, what-have-you). The potential reward to them for getting influence is so great that they'll invest tremendously in trying to do so.
The idea that "public servants" don't magically become saints, but respond to the same incentives as do the rest of us, is the core of public choice economics [1]. There is, in fact, a fair amount of research and literature on the topic from an academic perspective.
From where I sit, if you recognize the problem as one where we're giving the corporations and special interests, etc., a huge potential payoff by finding ways to co-opt governmental power, then increasing that power to try to combat it doesn't make sense: it's just increasing the size of their potential payoff. It sounds like silly pop-Zen, but the way to get such corruption out of government is to limit the power of government.