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.
I keep seeing this excuse, "If you don't like CSS you must not understand the box model". The box model is easy to understand but it still doesn't make CSS good.
This is more like saying you don't need to learn algebra and then saying you hate math because it takes too much work to get the answer you need. A surprising number of developers are under the mistaken belief that not learning how the web works is saving them time and over the years I've been amazed by how many people will instead unquestioningly spend years playing bug whack-a-mole with megabytes of JavaScript sturggling to replicate built-in behaviours.
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.