The problem with having something like +1 for 14 and +2 for 19 means everyone will have a 19 or 20 in their Prime Requisite!
This is not necessarily true.
We all can agree that hitting is very important. But even so, it is not so important that I would sacrifice everything else in the world to get it.
If I can get another +1 to hit for 1 point buy, I would do it. If I could it for 2, still do it. But for 10....no, that can buy me a lot of other abilities.
So there is a threshold when another +1 to hit just isn't worth the opportunity cost of all the other things you could pick up instead.