Why is Illaoi rewarded so much for remaining in one area?

Nagakabouros is assured of her motion, but wishes to test YOU to keep moving.

The longer Illaoi remained in one place, the intensity of her soul and nagakabouros is bigger. Think'it like heat: The longer one thing remains on fire, the hotter will become. Of course, if you remain on the fire too long you're gonna burn (Lose the game :P)

Keep in mind that illaoi is pretty much testing you while you fight her,so she wants you to move a lot and punish you for staying in one place too long.

Irony.

Cuz She is a Pussy. [Draven-trollface]