I can understand that. The biggest reason for this is probably that most AUs are either:
A: Concepts/Written stories. It usually takes less time to read a story than it does to complete a game multiple times, so there’s that.
B: Unfinished fangames. These tend not to have too much content in them, and definitely take less time to complete than completing Undertale multiple times. And even ones that are intended to keep being worked on need to have enough stuff in them to incentivize people to keep going, hence having a bunch of story packed in.
It’s a fair opinion to have.
I’m honestly mostly into AUs for the music though, since I don’t have enough time to read through anything and I can’t access my computer often enough to play through a fangame. If I did, though, I’d probably play TS! Underswap. From what I can tell it’s been orchestrated really well, even though it’s unfinished as of now. I’m pretty sure Undertale Yellow is also nearing completion.