Nasus is vulnerable to kiting and cannot really afford to itemize for percentage armor penetration. As a result, he is strongest during duels and skirmishes (where proper kiting is rather difficult) and before enemy champions have built armor.
Due to these factors, Nasus has an extremely strong mid- and early late-game presence and gradually falls of as the game progresses. While his Q keeps getting stronger, the enemy team has been able to itemize to handle you and properly kite/peel you to death.
Which hardly matters if your Q does literally half the health of a turret -_- He isnt suppose to team fight, hes a split pusher!
Some games you can't afford to splitpush and you have to teamfight (typically those games where your team will force a fight or get caught out before you get anywhere near and enemy turret consistently).
Sometimes you can't actually splitpush against a champion on the enemy team (i.e. Master Yi, Mundo) without them chasing you down and killing you, so you have to teamfight.
While it isn't always optimal to teamfight as Nasus, in some cases you have to.
Actually that has been a well established fact for a while now. Nasus is god late mid game, but isn't as strong late game.
This is because he isn't the only one getting stronger. ADC items scale in a multiplicative manner, while Nasus's stack scales linearly. Along with the fact that damage items scale better than tank items (unless you are Mundo or Kench) means that Nasus will do a bit more damage but in return take a lot more from the carries on the other team as the game progresses.
53.17% @ 35-40 then 51.38% @40+ you are looking at a 1.79% difference... i wouldn't stress lol.
In my experience Nasus is best at 20 minutes ish when you have a tank item,triforce, and completed boots. Nobody can kill him even in 3v1 situations. This brief power spike is about until 35 minutes ish. This is assuming your stacks aren't bad :P
It's because good players can make it hard for him to reach them. Endgame is all about cc, kiting, Mikael's, QSS, and high damage. If they have enough of that stuff, he kinda can't even get to anybody enough to accomplish anything, even if he does have more stacks.
Passed 30-35 minutes depending on total kills, he starts becoming weak.
not that big of a difference to consider it