MP4 works fine, but different ways of creating/editing/preparing video all have different ideas about how to produce a video file, and the people operating them don't always have the technical background to even know there's a difference. YouTube offers a simple solution: drag any video file in any weird format to the upload modal, and you never have to think about it.
https://blog.youtube/inside-youtube/new-era-video-infrastruc...
You're right that all the major video hosts offer it too, and I think GCS/AWS/Cloudflare all have APIs for it, but YouTube does it for free without requiring you to have any clue about all this stuff.