Right now that's YouTube and TikTok.
If I want to find videos on a specific topic, I use a search engine. DuckDuckGo yields much better results than YouTube, even when the results are hosted on YouTube itself.
It’s fine if that’s not what you want, but people do love it and it keeps them going back to YouTube and TikTok.
My wife and I watch ~15 hours of YouTube together each week, entirely on our Apple TV. If someone we watch were to move to another platform, we just wouldn't watch them anymore. Honestly even if there is an Apple TV app for the other platform, it's still unlikely we'd switch over just to watch them instead of filling up their "time slot" with someone else.
I've told the creators that I follow that I'm paying to enable them to do what they do, I don't need any more return than that.
I tried Nebula and others, honestly the content was either unappealing, low quality recycled material, or it took too long to find something interesting. The variety of YT is important and I'd also say 30% of my selections are based on duration, fitting the content into my lifestyle as well as my mood.
Content creators prefer YouTube because it has more users, and each creator is afraid that their followers wouldn’t follow them to another platform. Even content creators focused on open source or self-hosting kind of tech.
Honestly, I really wonder if users would refuse to follow creators whom they like to another platform. Are most people really that adverse to just watching videos on another website?