Notice how all the other sustain champions have heavy CC or general utility. He can meatshield, make the carry a ghost, and slow. With your logic, Mordekaiser needs a nerf too because he can STEAL your carry, has good sustain, and can't die because of his shield.

Morde did get heavily nerfed. He was nerfed specifically because he could not be pushed out of lane. Morde has less cc than yorick, he has no slow and has to get rylais for it. Needs to kill a carry with his ulti for it to be effective. Admittedly this is an amazing game changer if successful. Morde has to build for early sustain because of health costs and with the WoTA nerf this got hit again. Yorick needs nerfs for the same reason.

Yorick has a spammable slow, has built in sustain, doesn't need to kill a carry for his ultimate to be massively effective and does AD damage (slight advantage but when it comes to pushing towers it helps).

Morde has a late game. Yorick does not.