Why are so many people incapable of doing this correctly?

Not only does my inbox get filled up with loads of superfluous replies to mass mailouts where someone either

a) makes a 'witty' retort which isn't especially funny and is a waste of my time to read

or

b) replies to a question the sender was asking but no-one else needs to know the answer too

But far more annoyingly I'm currently trying to arrange a meeting and all five people I'm trying to meet with are seemingly incapable of using the "reply all" at all and keep replying to me only, thus meaning that it appears to everyone else no-one actually wants to meet and it's pretty much impossible to reach a consensus of when or where to meet.

Shouldn't there be some sort of training course when people buy computers or something?