My working theory is that OpenAI is the 'moonshot' kind of company full of super smart researchers who like tackling hard problems, but have no time and effort for things like 'how do we create an UX people actually want to use', which actually requires a ton of painful back-and-forth and thoughtful design work.

This is not a problem as long as they do the ChatGPT thing, and sell an API and let others figure out how to build an UX around it, but here they seem to be gunning for creating a boxed product.