Actually, Sesame Workshop owns the distribution rights to Sesame Street. HBO Max owns the streaming rights, but I don't think HBO or HBO Max own the rights any more than PBS ever did. HBO may have authority in blocking fan uploads, though.
Sesame Workshop owns the content. HBO and PBS have distribution rights.
I don't know why HBO doesn't go after the YouTube accounts either. HBO seems to have left the policing of this in Sesame Workshop's hands, and (beyond your standard content match software) they just don't have the resources to go after it the way HBO would go after someone uploading full episodes of Game of Thrones or whatever. It's really enabling those who think they have a right to store this content on their own channels to the point that when they're not getting a constant flow of new material to feed whatever it is they think they're doing, they go into a hissy fit.
And I get it; people are upset that they can't see the classic episodes that others have, but they really need to find something else to focus on and move on. They're literally hurting the way the industry sees the fans when it comes to this stuff, and they're shooting themselves in the foot in regards to the future release of more content they want to see.