I think bevy ui is the best example to give, it's like nobody ever did a ui framework with a pure ECS before. You can conclude either that's because it makes no sense to do that or that's because nobody has ever came up with the right way to do that. The bevy community thinks it's the latter.
It's especially concerning because they constantly talk about the editor, even though they don't even have any of the fundamental pieces for a gui framework in place. In bevy ui there is no way to create a reusable ui component, there are ways to do it but they all suck. So it's not even a matter of a lack of widgets or something, the problem is you can't even write a reusable widget, there is no foundation for a ui framework, and there isn't even a real plan to change it because nobody knows how to write a gui framework with a pure ECS.
But even if they figured that out, things like text input fields can't be properly implemented because there is no proper text rendering engine in bevy so they have to rewrite that first. Except that all rust text render/layout solutions in rust (it has to be pure rust because wasm/because it's rust) are still very experimental and immature.
It's a huge pain in the ass, people write games in rust because they want to write rust, not because they want to write games.