Storage and maintenance. OSes are miniscule in comparison to the data YouTube stores, we're in the multiple exabyte range here. Someone's got to pay for it somewhere. Floatplane might be a decent comparison as to what a FOSS YouTube might look like - they have a dedicated dev team and charge per channel to view, following more than a couple of creators would become cost prohibitive for me personally.

You absolutely need storage in a P2P network, the data doesn't just magic into existence, not only that but if there are insufficient peers in the network then you're not watching the video, smaller creators and older content would likely suffer as a result.