I think the problem is that developers do not understand that CSS should be treated like other languages, and developers should organize it rather than let anyone write it as they wish. You don't write your server code in a single file without functions, using global variables and introducing side effects, you don't let a trainee edit important code without reviews, why do you do it with CSS?
I remember reading that Github had something around 9000 CSS rules on a single page. Github developers are talented, but somehow they didn't pay proper attention to CSS code too.
CSS code should be split into components, should be easy to analyze, and should be written so that minor edit of one widget won't break something else on another page.
> In fact the seperation causes so much mental overhead, that we need interactive browser dev tools just to be able to find what styles are applied to what element.
That means CSS is not well organized.