I'm not good at this game so I assume I'm wrong, but I'd just like it explained to me

How come Cho's ult does so much damage in true damage and he can stack infinitely? I played today and his ult did about 40% of my health in true damage when he was level 7 and up to about 60% when he was level 10-11. And then even if he dies he doesn't lose any of the stacks he got and he just goes on forever, I don't get why this is a thing in the game when Cho seemed to be about risk/reward.

The specific game I played I'm 100% sure there are things I could've done better, I'm just wondering about why he's now/still designed like this.