Why is kennen's ult So strong?

Played a game against him, I was mid, he was top. I went 5/5, he went 3/0. He ganked from top and I was like okay w/e. He hit me with his ult and it killed me. Like he hit hard enough to kill me with just his ult? Then even later on when I built and He still cut through me like nothing with his ult. I should have also built , but still, the damage level of his ult is a bit high. Maybe some ap items need nerfing in the damage department. Some items have like 100 ap. that is a bit much. And no not all ap champions are without mobility.