Because it's not an alternative to people with YouTube as a job. It's great if you want to have a couple of videos hosted and watchable by others. There is no way to monetize them by their very definition and mission statement. Their own website says it was "created for non-commercial purposes". It's his job though, so he's not the target demographic.
You could integrate sponsors, which he doesn't do on YouTube either. Or redirect to patron or similar services.
There's also no (or very little) discoverable for people who watch sometime similar, which YouTube actually does extremely well. So how do you grow your audience?
If he only lives on Adsense, sure. But even then, if he posted to PeerTube as well, would he actually be missing out on Adsense money, considering the few viewers in PeerTube atm?
@[email protected] needs to sync his content. It shouldn't cost him much (if anything). If he has viewers willing to watch him on peertube, but they simply can't and they are forced to watch him on youtube. You can't build viewers on peertube by waiting for others or not uploading it to it.
That's basically the case for everbody on youtube going "but there's nobody on peertube". Yeah, it'll probably stay that way if you don't upload to peertube. That's a self-fulfilling prophecy.