> I wonder why he stopped making youtube videos.

I've listened to plenty of podcast episodes with CGP Grey and he wasn't secretive about the fact that he does not really enjoy work that much. He also maintained that once he quits he won't make any announcements, but rather just silently disappear.

I assume he has decided that he has enough money and retired.

He (or someone else) is still changing the old video titles and thumbnails to optimise views, though. This particular video used to be called Humans need not apply.