why is a renekton with no items doing more dot than mages?

Probably because if he has no items, it's early game, and Renekton is supposed to be insanely strong early. That's literally his entire concept. He's like opposite-Nasus. He's basically worthless late game, but if you fight him early, he's gonna win.

1300 in 4 seconds. i cant do that on lets say kassadin who is not tanky until i have 4 items....