YouTube has implemented hair trigger "anti-bot" measures that I suspect strictly limit the number of session cookies issued over a sliding window.
If you browse in Incognito Mode it does not take much casual browsing to exceed this limit.
It accuses you of being a bot and demands you log in to view any content.
A great excuse for a scheme to force everyone to log in and be tracked.
Has been like that for a while. I don't think it's as much about forcing log in and more about monopolizing access to YouTube videos for AI training.
It's much worse if you have a VPN.
I tested this at Skipvids.com