Might as well follow up with "why are desktop apps so badly designed...?" Web technologies are certainly their own specialization, but UI/UX is bad for a most user-facing software, regardless of the technology used to present it.

Web technologies don't make UI/UX harder (in a lot of ways, they make it easier!). There's just a lot of them, and they've become dominant for the vast majority of things the vast majority of users experience.