You can step on one of them. Then you realize that you stepped on one, and there could be billions more. You can't check all of your walls for them with sheer brute force without destroying your walls and making your house unliveable.
Then you also realize that they've been around since the dinosaurs and are pretty much impossible to eradicate entirely, since they keep adapting to different conditions. You'd have to destroy all of the buildings and houses, which may or may not harbor a roach, but you'd have to destroy it all to be sure.
You can, however, eradicate the source of a particular nest that's bothering you by hiring a professional to stop the eggs (the root cause of roaches), not the roaches once they're running around.
Good analogy!