Yorick is like a perfect storm of bullshit. Let me explain. Yorick's kit revolves around him summoning ghouls to attack and weaken your oppoent for you, so you can eventually whittle them down and kill them. The problem is that it is very feast/famine. It's the Gangplank problem, either you run out of health, or he runs out of mana. And by the time Yorick builds tear, he gets to ghoul spam indefinitely, and becomes impossible to engage. You can't all in against a Yorick because it's basically a 4v1, and you can't poke him down because of his sustain. He literally has no counterplay. And by the time he hits 6, even if you do manage to kill him, he ults himself before that happens, and then takes you with him.
After the laning phase is over, however, he brings very little to teamfights, outside of ulting the AD carry. The best way to describe Yorick is "win lane, lose game." Because he ranked #1 on the list of most frustrating top laners to fight against (to put this into perspective, Teemo was like #2 or 3) and because he's anti teamplay, Riot said, and rightfully so, he's a failure in every possible way in terms of good design. And that's why you can't play Yorick unless you buy him with RP/IP. I didn't get Yorick until the Karthus VU when the Pentakill bundle came out, but I can safely say that even to this day, Yorick still hasn't changed as the textbook definition of toxic champion.