Because evil presidents either turn into dictators, or get voted out of office early. Poland had a very backwards (not really evil, but clearly not suited for the office) prime minister some time ago. For about two years. After that, his own government disbanded itself. Most people in the government do, at least for some part, actually care for what happens to the country, even if only because they live in it. An evil person who somehow gets elected (for example, because of a major crisis) will get unelected unless he/she finds a way to stop the elections completely.
In general, I noticed that politicians are usually motivated either self-interest or an actual desire to make things better, at least from their own perspective (even if they're too incompetent to actually do so). Usually a bit of both, really. Problems arise when their version of "make things better" is backwards and detached from reality, when they put their own interest before that of the state, when they're just plain incompetent, or, perhaps the most commonly, all of this at once. But actual malice and trying to deliberately screw with the populace is rare.