Why isn't there a video-only Twitter?

There was. It was called Vine. Twitter bought it before it launched, ran it into the ground (despite huge cultural influence), and recently decided to revive it under Musk.

Vine was very likely the inspiration behind TikTok, and TikTok is definitely filling the void that it left.

https://en.wikipedia.org/wiki/Beme_(company) tried I think. 2-8 second videos, only direct recordings (not editing like on tiktok).

I want to say TikTok but it has text comments. Seems to me the friction in posting a video would deter comments, but the comments it deters might be the ones you didn't want.