Because with celebrities come fanbases.

Imagine if whoever the new hot artist is put out their next music video exclusively on Peer-Tube.

Suddenly millions of people would be using peer-tube. Then they'd ask "what is the fediverse?"

If you want to keep the fediverse small and isolated, go stay on hexbear, or whatever that one isolated instance is.

I would rather every single human be using the fediverse.