The short answer is that not everyone has a [[USGA]] handicap.
The longer answer is that if we required that everyone have a [[USGA]] handicap, we’d never get to scoring the rounds. It takes 10 rounds to establish a [[USGA]] handicap, which is more than half the golf season.
Another option would be to not use handicaps at all, but that wouldn’t be fair to everyone, and only a couple of people would have a realistic chance to get into the playoffs at the end of the season. Where’s the fun in that [[:-)]].