It's because it's a tremendous amount of time and work and effort and QA to get UIs to look really really well-designed. (Even then, the design can still suck, like modern Windows or appleOS 26.x.)

I don't think people realize just what an insane amount of labor it is to get these things implemented, even if you're handed a perfect design spec up front.

Maybe LLMs will close this gap once they get better at seeing things.