Your use case is simplistic so it dismisses actual use cases.

People make money on youtube through ads, you can't do that (as effectively) on your own server. This also ties with the analytics.

Some organisations like the ready-made administration solution. Uploading files through ftp isn't for everyone. Youtube (and hosting platforms) has a nice ui to manage all the content, handles the user authentication etc.

Bandwith.

Backups.

I aggree that for people who don't need all these, and are tech savvy, uploading an mp4 to a server is the way to go.