I think a central reason is that CSS was originally intended for laying out text instead of laying out web applications. So what many programmers do/did with it was something that CSS was not originally intended and thus not optimized for, i.e. a lot of people use(d) (out of necessity) CSS in a very "hacky" way.

> I think a central reason is that CSS was originally intended for laying out text instead of laying out web applications.

Not just was. Also is.

"Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g., fonts, colors, spacing) to Web documents." https://www.w3.org/Style/CSS/

Such confusion is entirely the fault of insufficiently logical programmers P;