The problem with the default UI is this:
It was made over a decade ago. It fitted the theme of the game and fulfilled the brief (usable and helpful for a large majority of players), but as soon as the very first groups of people started to play in Vanilla endgame, it became clear that while the default UI worked ok for basic gameplay, for advanced things it fell very short. Over time, WoW has become an endgame game, and that means that for a big chunk of players the default UI does not meet their needs.
This is something that Blizzard has never had to fix because they had the fantastic foresight to make a flexible addon system. They didn't need to improve the UI because the community could do it themselves. Over the years some addons and some community made UI alterations have become part of the default UI. The current raid frames for example are a vast improvement on the Vanilla frames. Core frames being movable without the need for an addon. Spell alerts. etc.
However, the basic design of the in-combat UI has barely changed in 10 years. That's not to say the Blizzard has done nothing to improve the UI in that time; much of the non-combat UI and panels have seen a lot of changes. Even simple things like search boxes are vital modernisations from the clunky original designs. But it has become clear that Blizzard's policy is to not update the combat UI, or the overall look and feel, at all. Perhaps they think they nailed it first time 10 years ago. Perhaps they want WoW to look like 2005. Who knows.
For my part, the reason I love changing the UI is because I think that the UI of a game is a vital part of how it feels to play it. The UI is not in the background, it's not an afterthought. When you play other games that have come out in the last few years you can see that those games obviously employ some really passionate and dedicated UI designers, and it's a stark contrast to what WoW has. The interface can be a part of what inspires you to play. That's why even when I'm not subscribed, I still regularly check this forum. It's up to the community to provide great modern UI designs, because Blizzard has all but abandoned it.