Not only pedos, in general sadistic people tend to try and get jobs which would give them the feeling of power over someone, and not all of them can be dictators, warlords, just politicians, even lowly prison guards or policemen, also cowardice is a factor. So - child-related jobs.

But, to be frank, I'm not sure background checks are going to do that much good. People of this kind tend to bunch together, help each other, and can either get past the radar rather easy or utilize these checks to discredit anybody who'd be a threat to them.

It's a complex matter.