On that same vein then Wally Mk2, why do we bother we Police? Criminals still manage to commit crime anyway so why bother? We still see dodgy vehicles on the road that somehow passed a roadworthy so why bother with those either? Terrorists, drug smugglers and other nefarious fiends are going to find ways to get things onto an aircraft they shouldn't so why do we bother with that?
I know I'm going a little overboard with some of those examples but the fact remains, just because something isn't 100% effective by itself doesn't mean you don't bother, it becomes part of the overall safety net designed to try and make sure people are in good habits and capable.
You're entirely right with the AFRs, someone can pass one and go out and do something exceptionally stupid, in that case the instructor turns around and says "Well, I told him not to be a complete tool, he demonstrated he was capable of being a safe Pilot so I signed him off" Yes, it's a bit of ass covering to some degree but there are plenty of practical aspects to it, doesn't mean we don't do it just because people can go and be twits after the fact!!