And then the forums and subreddits are flooded with miserable folks complaining about how destructive, inextensible and unpleasant to use those experiences are. This is not the problem of UI in engines itself, it's problem of how long it takes to bring it to acceptable state with all those moving parts. For UE, Unity and Blender it took decades.
Complaining is a lot easier than actually doing anything.
Realise that the people you are slinging mud at are actually busting their asses to provide game engines to the public some of them for free and with the source.