The dicks want to control because they like to screw with people, and the sensible people don't like to screw with people, so avoid being in charge.

The sensible people are lazy or to busy to take part

Apathy and ignorance.

Why? Because it takes a sociopath with more or less psychotic tendencies to climb over all the bodies to get to the top. But that's just my ironic view of our current crop of incompetent world leaders; there are exceptions, but they're few and far between, as we can see by the mess the planet is in. If the Earth was a corporation, we would be bankrupt. LLOL

George Carlin says it best:

Because when sensible people are elected, they become Dicks ?

Because unlike many sensible people, dicks are not bound by ethics or conscience and harbor little to no guilt, shame, or remorse about lying or exploiting others to achieve their damnable goals. Also, maybe there aren't as many sensible people in the world as we hope there are. Also, it's tough to be sensible 24/7. That's why one should be vigilant about opportunistic dicks ruling the world.

Because maybe we only flatter ourselves that we're sensible. If we were sensible would we really allow Dick (capital D) rule?

Because dicks love power, and will do most anything to get it. And they do.

Because "dicks" insist upon inserting themselves into everything. Lol. Don't mean to denegrate the seriousness of your question. I just couldn't help myself. Sorry.

Sensible people are intelligent enough to know it's almost impossible to get away with murder.

Because it’s (usually) illegal to drown Malignant Narcissists