This is incredibly funny considering AI generated content is currently endemic thrash on YouTube. AI generated fake trailers, universally hated zombodubbing, weird "touches" on shorts…

Which to be clear isn't a contradiction to your comment at all. It'll take work and time though, at minimum.

Why the AI-wielding creators would choose to use a new, different service, and not an existing service with a colossal audience?

In a theoretical future The Algorithm would include content generation so that the platform can generate content for you instead of just suggesting it. Could apply to TikTok, Spotify, etc. if the generated content is good enough.

Hundreds of people are probably building those at the moment. The more relevant question is why would anyone watch it?