I agree that the UX on Tiktok is cleaner and, like you say, in part part that's due to it being only short-form. It's worth noting that "short form" here means up to ~10 minutes long at this point, includes live videos and also includes photo galleries.

But it's more than that.

When I started using Tiktok, Charli D'Amelio was the biggest creator I believe and not once did I ever see one of her videos. I'm just not in that demographic. I've had repeated experiences on Tiktok where I'd see a new creator and see they have like 17M followers and I'd think "how have I never heard of them before?"

The way I describe this is that Tiktok's content is effectively segmented and isn't "global". By "global" I mean if someone is a top creator on IG or FB or Twitter, you'll see them. The platform will push them out to you and Tiktok is just more sophisticated than that.

The second big difference is the responsiveness. It takes other platforms longer to learn. Maybe they've gotten better now but, from what I know, historically other platforms had daily jobs that updated user recommendation preferences based on your activity. So if I started watching a lot of gaming videos, this wouldn't be reflected in my feed until the next day. Tiktok I think was the first to have a truly real-time updating feed.

Now this isn't a straight real-time vs overnight situation. It is/was more hybrid than that. So in FB's case, recommendations were more real-time but updating your preferences wasn't.