Because many web sites and apps aren't as simple as "my first homepage" and don't only consist of first-party code. Think component libraries. Reusable code. Content management systems. Third-party SDKs (chat widgets, support widgets, payment widgets like Stripe, etc.).

One of my earliest webdev jobs was at a company whose product was a widget you could add to your site by adding our `<script>` tag. Thus, our CSS needed to coexist with the first-party site's, not to mention any other third-party widgets on there. In other words: the same exact reason you need modules in traditional languages.

It's true in a case where you are doing the described thing you will need to come up with your own module system and ways to not step on other people's stuff but it isn't actually difficult. Although I have noticed some stories recently were quite big companies evidently didn't put in the work to keep from messing up other people's stuff.

Of course one drawback with that is you are also depending on developers and content managers at sites following your documentation on how to use your products, which is a different problem.

on edit: obviously if you have been writing css since 1997 and one of your first webdev jobs was this kind of thing, things were much more difficult back then. I did the same sort of thing in 2014-2015, not particularly difficult to make work. I worked web dev since 1999, first job was dynamic generation of web sites and other media from single source data.