Probably because very few learn CSS right. In particular: the box model.

All the devs I know that struggle with CSS don't understand it. The ones that don't have issues have internalized it and don't need to think about it much anymore.

The box model is one of those fundamental building blocks you really should be learning very early on, a bit like an "if" statement in imperative languages - you can hack around it with ternaries and single-iteration loops, but you're never going to be as effective or have as strong an understanding of the language as you should.