It's because you can't control it. There are limitless attackers and vectors. Security is mostly a game of being hardened enough to where most of those attacker

It's because you can't control it. There are limitless attackers and vectors. Security is mostly a game of being hardened enough to where most of those attackers will give up and go off looking for easier targets. Against a zero-day that nobody knows about yet, or an extremely determined attacker with a lot of patience? You will eventually lose, and you have to do your best to detect when it happens and act accordingly, as stated previously.