Same reason there are still websites that heavily rely on unstructured code heavily using jQuery.
Every little thing you add to a project increases its complexity. That can be worth it when the project will otherwise become less maintainable or harder to reason about in the abstract, but sometimes it's not worth it.
Cheng Lou gave a brilliant talk on this problem at React Europe this year: https://www.youtube.com/watch?v=mVVNJKv9esE
The talk is very abstract (he even jokes about that observation in the talk itself) but I think it's very applicable to this and similar questions.
The other obvious factor is that not every person writing CSS (or JS or anything) is a professional designer/developer doing that every day as part of their full-time job. To those people tools are something that gets in the way. If you understand just enough to know how to make changes to a few files and shove them to the server over FTP it's a huge step to add even the most basic tooling. To clarify, in order to use proper tooling the way most people expect it you need:
Switching from "just edit this file and throw it on the server" to THAT can be daunting and it requires actually taking the time to learn and understand all the moving parts you are adding (and why they have to be there and how they still make your life better despite seemingly providing no direct benefit right now).
I'm not saying people shouldn't switch, but with real humans working under real restrictions like deadlines and having a life outside work... I think it's at least understandable that some stick to the devil they know.
EDIT: But really, just use Webpack and CSS modules. The future is here and it's brilliant.