It's almost certainly a network effect/chicken and egg problem. Everyone has YouTube, so putting your videos up only on another platform is sure to break the bank. You could try to really push your audience to another platform, but generally a large portion of your audience isn't interested enough to download a new app or go to a different website to watch just your videos. There would have to be an extreme mass exodus with nearly every prominent creator switching to a new service for most people to make the effort of getting a new app and creating another account.

There is also the problem of any YT competitor needing an extreme amount of money to invest without any promise of return for a long while. Google has invested 20+ years into digital advertising, so unless they use Google ads themselves they end up needing to roll their own ad platform. PR, managing the content creators put on your platform, etc. are also huge mountains to climb, which is why YT continues to struggle to "appease their creators".