They have a lot of videos. You're statistically unlikely to care about any randomly selected one. By watching a video, you establish that it's interesting enough for you to watch. It's much more likely that you'll want to rewatch it than that you'll want to view a random video.

I'm only half-joking here. To me, YT algorithm seems to be a mix of "show random videos you've already watched" + "show random videos from channels and users you watched" + "show the most popular videos in last few hours/days/weeks". It's pretty much worthless, but what are we expecting? Like all things ad economy, the primary metric here isn't whether you like the recommended content, or whether that content challenges you to grow - it's maximizing the amount of videos you watch, because videos are the vehicles for delivering ads to you.