Evidently they don't keep all of them. 😉

As a thought experiment, do you think that the ad revenue from watching ONLY videos on that guy's channel were enough to cover the operating costs and infrastructure required to store and host all of that channel's videos? If so then it should be possible, if extremely painful, to operate a vlogging business outside of YouTube.

Getting users TO your personal vlog site is a whole other problem but through Patreon (as an example) it is possible to funnel users out of the Alphabet/Meta/ByteDance networks. If any of my favorite channels were to announce they were transitioning to a new video sharing network I would gladly follow them off of YouTube.

They simply have a massive monopoly, and it is impossible to replace them, no matter how good your platform.

Because you depend in them and their buddies for advertisment.