You’ve got it backwards. That much gets uploaded because it’s unmoderated. The vast majority of it is junk that everyone knows would never pass any sort of quality filter, so no one would bother to upload it.

That makes no sense. YouTube has no policy against "junk" videos, there is no reason why individual users would change their upload habits except for a tiny minority that knowingly upload violating videos.

No, there is that instant dopamine hit of uploading a video and seeing the likes in real time. Whereas if it was, upload a video and 6 months later it might be approved, people would be more diligent about it. This is very obvious human nature.