why don't they?
That's the subject of another thread, if you want to start it. The fact remains that if someone isn't taught how to do a thing, and you look down on them for not being able to do it, that reflects poorly on you, not on them.
In that other thread, if you should feel moved to start it, you might consider what you're really trying to accomplish and whether teaching people to punch is the best way to do that. I spent a lot of time in karate learning how to punch, but that was part of the point of that style. If you're studying a style where the ability to punch is incidental, you might ask yourself whether there's a better way to achieve your goal.