Same reason you see Netflix's homepage.

When logged in, Youtube.com shows you a mix of new videos from creators you watch and related/recommended videos in the effort to provide you something you will probably want to watch. So I go there when I'm in the mood to watch something, perhaps while eating dinner.

And it works. If you want to see how ineffective it would be just looking at a list of popular videos, view the horror that is the trending tab.

https://www.youtube.com/feed/trending

This is a great question and something I've often wondered. Most of what I see after deleting recommendations, likes/dislikes, and subscriptions is just click bait. If I put something on that is educational for my kids, next thing I know it is recommending vile content to them. Where do those suggestions and recommendations actually come from?