Putting an annotation on a method is much easier than registering this method at n places in ordinary code, which could look any number of ways. If you move to a new project, in the first case can just immediately jump into framework-related stuff, while in the second all this logic has to be untangled first. And let's be honest, Spring/Quarkus probably got their abstraction right after n iterations, which is not necessarily true of a random team.