Politics as a career generally attracts weasels. It's that simple. To me, the stature of just about any position one can hold in politics is significantly less than pretty much anything else one can do to make an honest living. Regarding US political positions, you don't have to be exceptionally bright to be a Senator, Congressperson, or even President. You do have to be driven, and unfortunately, many of these people are driven either by a huge ego, greed, or a Messiah complex.
This is a neat classification. Of course there quite a few of decent politicians too.