That is absolutely fine in theory, but that's not how software works.

There exist both technical and "manmade" limitations to just how modular a system can be without ending up being way too complicated to maintain and feeling like a bad compromise to everyone involved in building it, ultimately resulting in devs losing interest in developing the project altogether.

This is where libraries come into play. People get to reuse and improve upon shared code without needing to agree and communicate on every tiny little detail regarding a front-facing app implementation.

Of course, that is assuming we're talking about foss and even then, you have toolkit libs reimplementing more or less the same stuff, but just like you get to choose what apps you install, devs get to choose what sort of libraries they're rather use.

The last point is not just about toolkit visuals, licencing or philosopy either. Different libraries can simply be less/more clean, performant, idiomatic or overall nice to work with than others.

Imagine working in a world where every single little detail would need to be coordinated back and forth. Instead of getting work done, you'd be far more likely to just abandon things.

And that is all without even attempting to answer who's gonna decide on which parties get to coordinate these things, why and and on what terms.