Rather: fake youtube followers have a negative ROI.

Perhaps because they don't matter, but perhaps because they are more difficult to create or Google is better at spotting fakes.

Or maybe search priority is related to percentage of followers that interacted with a given video

Or maybe Twitter and IG dont care about fakes...

That's my guess. On twitter/ig, advertising deals would be done directly between accounts and advertisers, so the social media provider itself wouldn't really care; fraudulent engagement numbers are third-party. YouTube is different, where the host also provides the advertising service, and fake numbers hurt Google's bottom line. So they're probably both harsher on fakes and better at detecting them

A large percentage of followers that don't engage in watching videos hurts you in the YouTube algorithm. Twitter and Instagram likely just want to show high user numbers.