Because this game is based on ten year old concepts, where hats replace your hair, instead of sitting on top of them, and apparently it was too hard to do something about clipping, then it was to make something "good enough".
Fallout 4 came out in 2015, and they figured out how to separate from hats. But a game that was launched 5 years later couldn't figure it out.
Yeah, it just makes me not wear hats as the hair style is so drastically different than what my character has. Also the hat hair doesn't show up in mirrors and so you appear bald which is equally as goofy.