You could say the same thing for any abstract mathematical construct, e.g. metric spaces. There's no theorem (that's not itself about metric spaces) that can only be proven using metric spaces. It's always possible to make new abstractions (simply take the antecedent of any theorem), so whether you adopt a new abstraction is a matter of taste and fashion.

In Grothendieck's early work on abelian categories, categories are themselves the objects of mathematical study, and nontrivial results about them are established.