What has the amount of instances to do with that? Everybody can open one, that means nothing. And what means "figured it out"? It don't see massive traffic there.
The largest instance has 20k users (and signups closed). youtube has 2.7 billion active users. now, this instances does not give any information about the used hardware, the second biggest (AntTube) does though.
Looks like its running on rather low-spec Core i7 and 8 GB RAM. The site does however say nothing about how much the hardware is utilized and also not how much bandwidth is used per month. So what are the cost here?
And more importantly, how does it scale up? How much would PeerTube need to serve 100k user, 1 Million? 100 Million? Is it optimized for that?
You massively underestimate the development effort and the know & how that is in Youtube.
And don't even get me started when it comes to making money with it. Some companies will not just advertise on some random privately run sites. Apart from the fact that they are legally probably not even allowed. There must be a real company behind it with which you can conclude a contract and which can be held liable if something goes wrong.