Primarily because you're not supposed to help out every single time your man is beat because it leaves open a shooter (or someone). That's why these 'idiots' don't help. Help defence is only really useful when you're trapping someone or double teaming, not when you've been beat cleanly. Then, you're just covering for someone else's mistakes.
The only time someone should really help is if they see the guy attacking the rim one of the post players should come it and try to get the charge or commit a hard foul. But simply picking up the man because he got beat shouldn't happen.
Again, I've said it before and I'll say it again, defence is all about personal pride. You need to stop the man, not anyone else. It's YOUR job.