I think this network effect and the discoverability aspect are the main reasons why it's extremely hard to compete with YouTube. Why would people use another site if the content they want to see is not there, or is too difficult to find? Why would creators put videos there if they can't find a large audience?

The YouTube algorithm is problematic in many ways but it does succeed in viewers being suggested videos they want to see, even if the signal-to-noise ratio is not very good. That's hard to replicate when starting a new service.