I also believe those YouTubers are often making additional money from the compilation videos with sponsorships and affiliate links, in addition to what TikTok pays them.
Is it just a matter of TikTok hoping most small-time creators will see it as free publicity and/or figuring they probably don't have any legal knowledge or resources, or even awareness their content is being monetized by others? Or have the users signed away all revenue-earning potential due to the music licensing, or something?
If the latter's the case, it seems unfair that random YouTubers can monetize the content a TikTok user created while the user gets nothing from it, and while the user can't even monetize their own content themselves.
Cody Ko ran into this last year, video here (relevant portion starts at 4:40): https://www.youtube.com/watch?v=SDdR8cue4eE