Comment by presentation

This! Usually CSS doesn't even make sense outside the context of the page/component it's styling.

There's also some nice stuff with modern tooling, where CSS is global by nature but writing it with your JS allows for the platform to automatically namespace your CSS, making styling behavior much more consistent.

Separately it also often makes sense to style things not just based on the HTML produced but also based on data stored in JS; writing your CSS with your components allows for a greater level of dynamic styles (which, to be fair, if you can achieve with pure static CSS is preferable, but it's not always the case).