> when that could be left up to libraries?

Because those libraries will not materialize in time, and more importantly the hobbyists who are supposed to write those libraries don't have the testing capabilities of large organizations (e.g. testing across hundreds of hardware configurations).

...or worse, the libraries do get written, but multiple times in mutually-incompatible forms that are tightly coupled to specific compositors / desktop environments. (Screengrabbing, anyone?)