It seems that politicians make a lot of decisions that aren't socially optimal because they want money from lobbyists and other campaign contributors.

Or because they haven't a clue what "socially optimal" is. Or because they're just supporting their side. Or because the legislative process does not lead to socially optimal decisions. Or because they're personally indifferent to social optimality.

Why are congressmen so motivated to stay in office, make $175k a year, exert a minimal amount of real power, and spend their time catering to lobbyists and making socially suboptimal decisions?

Social status and fame. Hollywood for ugly people.