I'd imagine hosting costs for a video sharing service would be much higher than other parts of the Fediverse and they'd need a surplus of donations to be able to start doing that.
I think the channel membership idea is the better one as it would provide a steady amount of money but most big content creators will already have Patreon, so perhaps it is a little redundant.
However, I don't know what the breakdown of income streams is for big YouTubers but I suspect the cut they get from YouTube from ads accounts for a large percentage of it.
It may just be that we have to get used to the Fediverse being something we directly contribute to, rather than us providing the.money passively through viewing ads - we are no longer just passive consumers. So if you are happy with your Fediverse instance, throw them a buck a month, if you like a content creator of PeerTube then sign up to Patreon. It's a pretty small amount but if a high enough percentage of people do it then it should really help those building the Fediverse and populating it with content do so on a more professional basis.