I would suggest that it's not because their crooked per se, but because getting into office requires an extraordinary fundraising and PR effort that takes all their time. With the rise of finance and technology, the systems in our modern civilization are incredibly complex and difficult to understand, and the only thing legislators know about issues is what lobbyists tell them. There are no effective lobbyists for anything that is not making someone a lot of money, because it's really hard to compete with well-paid professionals.

So even with the best of intentions, the system conspires against legislators getting an informed yet neutral viewpoint on how policy will affect the myriad systems that make up our society. As if that weren't enough the anti-intellectual streak in America tends punishes anyone who tries to bring in more nuance and public research to difficult questions.