OK there's two reasons why
is terrible on Nasus.
- Makes it harder to farm stacks
- Completely unneeded - with 40% CDR your W will be slowing people almost constantly, it's really easy to stick to people already.
The reason you build full-tank on Nasus is because if you've farmed enough stacks then you will apply more damage to the enemy by staying alive via tank than you would by having more upfront damage. You're already hitting them for 1000 damage every 2.4 seconds and that's ignoring your E/R damage.
i couldn't stay afk in top lane
Who says you need to stay afk in top lane? You have teleport, as long as you're farming efficiently you have a chance to demolish the enemy team every 4-5 minutes.
If you want a high damage build on Nasus, the only real option is full-AP Nasus (Morello's, Ionian Boots, Liandry's, Deathcap, Zhonya's, Void Staff).
At the end of the day, though, why pick Nasus if you want a higher early game impact? Pick something with instant level 6 impact like Malphite, Wukong or Hecarim.