Because a lot of people don't want all the other stuff that comes with having a facebook.
If I see a facebook link, there are many problems:
- I don't know if it's a video
- I don't know if I'll see a login page or if I'll get to the content
- I don't know if the content is viewable by me because of privacy settings
And then this works the other way - why should I share a video on facebook when I can't tell if my recipient will be able to see the video? I don't know if they'll get a login prompt. I don't know if they have the permissions to see the video.
All this friction doesn't exist with youtube, or at least is extremely minimal.
And a small 10s commercial on a YouTube video is a small price to put up with in comparison to the rest of the shit that comes with being in the Facebook ecosystem.
Only reason I still have an account is to be in touch with some high school friends and that's something I barely do these days.
More like "I see a youtube video and click it, I get a 10s commercial"