Making sure that the top-level classes of components are unique isn't difficult. Making sure the inner classes are unique (yet still follow a consistent naming convention) is only possible with a methodology like BEM, which is clunky. And you want inner classes to be unique so that you can nest components safely without having outer components accidentally styling the innards of inner components.
How, exactly? Naming aside, splitting things into separate files is often useful, and making the browser fetch multiple files for the base stylesheet is always going to be worse than packing it into a single file.
Modules make writing CSS much less of a naming game. Anything you can do to reduce one of the 2 hardest problems in CS is worth it.