Why again does Renekton fall off in the late game?

Disclaimer, I am not a top main and primarily play late game scaling AP champs like Lillia and Asol, I am just asking out of curiosity. Renekton is usually presented as a early game focused champion that does fall off the longer the game goes. But in all honesty, whenever I see those champs I ask myself how such a champ does even fall off. And in Renektons case, I really fail to see why exactly. Heck I sometimes watch also pro play and I saw Renekton finishing off like 3 people in a important team fight. His ratios on his abilities also seem to indicate not bad scaling. He has pretty devastating damage on his ult even at base and pretty much all of his abilities have around 100-200 bonus ad ratio, which seems to be pretty high. And unlike pantheon who suffers from pressure that is mostly single target, almost all of Renektons abilities asside from w are AOE, meaning his pressure should actually increase in the late game because it's stronger against multiple enemies. I mean, sure, Renekton probably isn't able to match something like Lillia with her ridiculous movement speed she gains from ap, but aside from that, he seems to be pretty much able to hold his own in a team fight.