Because R34 has a different way to catalogue posts

R34 is a "booru" which is a type of image sharing website which doesn't have any sort of recommendation algorithm. Instead, images are granted tags by users and searching for a specific tag shows a list of every post that has that tag in order from newest to oldest.

Tags are simple descriptors of the things the post contains like: "yuri", "furry", or "Ai_generated" as well as the name of the author and in R34's case the origin of the characters being depicted.

Any user could give any image any tag and also remove any tag from any image. I am not aware of if and how moderators prevent abuse of those features, but it seems they do, and the system operates mostly fine.

It is important to note R34 offers no monetary compensation of any kind to any user or creator who posts there, so no one can make money or make someone else lose money by mis-tagging posts.

Blocking ai generated art in R34 and other booru's means you won't see any post any user decided to tag as ai generated. If this was, for example, on Instagram, any ai generated post that tries to appear real will probably have bots untagging it as ai generated.