Because people who want tiktok content watch it in tiktok, and those who don't don't like the format in general.
If tiktok started hosting half an hour long documentaries it wouldn't be any wonder that nobody would watch them, as the userbase doesn't have the attention span for that and they aren't scrolling tiktok for that type of content.
I personally have only one user whose shorts I watch, B. Dylan Hollis. And even there I would much rather prefer longer videos, but I'll take what I can get.