As it was already said, it’s fun. Because it’s almost like trying to solve a puzzle. A well thought out world has incredible internal cohesion and mulling on it reveals angles that aren’t necessarily even explicitly pointed out. Realizing that interconnectedness is immensely satisfying (even if you only have it pointed out to you) and makes you appreciate the work even more. Let’s say a fantasy world where there’s a well thought out magic system, and a well thought out financial system - and thinking on it reveals that the two are very interconnected, even if it’s never highlighted in any way. It’s a meta-story within the story. Popular fiction tends to have a lot of this, which makes it a popular topic for discussion, which makes it more popular. Brandon Sanderson’s works are a great example of this. One Piece is a great example of this. I’d say the satisfaction of interconnectedness is also why multiverse stuff is so hot these days, though most of it is executed in a fairly lazy way (just piling things on top of each other and handwaving anything that doesn’t gel as “well, it was another timeline”, looking at you Marvel).

Of course sometimes there are some works that ride on such nice vibes that you don’t really care that it doesn’t really make much sense. Harry Potter being a good example (or at least was before Rowling went off the deep end). HP world building is actually so flimsy it’s embarrassing, but it still managed to tap in to such a good idea of a fantasy that people were largely willing to overlook it.

Got annoyed with Reddit and the social media megacorps in general. I can actually sometimes remember and recognize usernames here. Reminds me of old school forums.