Money. Lots and lots of it.

Hosting video on a significant scale is very expensive. Stupendously expensive.

Convincing people to join is also going to cost a lot of money. Consumers are on YT because creators are there, and they are already used to the platform. Creators are there because the consumers are there. And there is a robust infrastructure to make a living from content creation.

Financing is especially difficult for such a project, because companies are willing to pay way more for targeted ads. For which you need some data about your users. The more data you collect, the better the and targeting can be, the more companies are willing to pay.
Assuming there are enough users for companies to pay for advertising at all.

If you want to work with the original project, you have to push to the server that controls the original project.

For each project there is one authoritative instance, one “server” that everyone pushes to. Otherwise you get chaos.