I think part of it is also that we're able to still LARP as full developers of complex systems while vibe coding by seeing an interface that makes us look like l33t h4xx0rs even though we're just pressing continue 15 times
Bad UI plagued software development since ages immortal. The reason is not AI. Good UI design is a skill (or art?) and not an afterthought. But most people do not see it that way and that is why things are the way they are.
Companies make UI/UX to prioritise first 30 minutes of the experience, to keep user using it long enough that they stick with it. Not the 8h/day work the UI will get when a tool become pillar of your work.