There’s really no reason for them to prefer the Fediverse over the centralized corporate platforms that basically cater to their use-case.
Wouldn’t a couple, maybe niche admittedly, reasons be less advertiser-influenced rules/moderation and in certain cases more control? E.g. YouTube’s notorious for its algorithms affecting views abruptly & near inexplicably, whereas something like PeerTube (or for streaming, Owncast) lacks those and enables less restricted content (fewer worries about automatic ContentID noise).
Similar situation with Pixelfed instances not having to fret over folks post nudity 'cause advertisers supposedly don’t like their adverts next to nudity in some regions. Don’t get me wrong, I see where you’re (and they’re) coming from on this, you go where the audiences are & where, give or take, certain features play to your benefit (i.e. recommendation algorithms), but I’ve also seen so many creators also chafing against the awkward antics of the corporate platforms, be it YouTube, Twitch, Instagram, Twitter, Reddit, etc.