This is an intellectually lazy and overly cynical view of human nature. The reason politicians seem so terrible is not that they were the dredges of human morals and character to begin with, but because a) power corrupts, b) the stronger the light you shine, the worse every flaw looks.
Do you ever make mistakes in your job? Things that, in hindsight, were non-optimal? Of course you do! And in most jobs that's not a big deal, but when you're a head of state, even a slight error in judgment can get you torn to pieces in the press. And since positive news isn't news, most of what anyone hears about a given president will likely be bad, so inevitably you've mostly heard bad things about what a president (or national-level politician) has done in the past. Hence, everyone seems terrible.