There is a strain of video (below) on there that is just like perceptual candy to kids. And there are LOADS of these videos in slightly different versions.
My own anecdote:
Our 2 year old found these videos (3 years ago): https://www.youtube.com/watch?v=V7IsNFDtG4U&list=PLHZrFxrJbt...
At first we were like "seems fine, colors and shapes...keeps him engaged". After a month or two he could just stare slack jawed at these videos for hours, it was like he was being lobotomized, and begged for them constantly. It was clear that they didn't actually teach him anything, and he couldn't even remember anything salient from watching them. We turned them off and haven't let him watch since. Even things like watching looney toons is infinitely better because there is some base narrative, characters, problems etc.