Why? That's precisely the reason Pantheon buffs do exist.*

The game has so offer some challenges to the player, dangers to face. There are a lot of unfun gameplay mechanics (snipe oneshots, on death effects, ...), but ground effects are tolerable, I feel. By removing ground effects, they'd have to add a different danger back into the game. And I'd rather take something one can play around by simply not walking into it (or taking defense layers to negate it) than whatever the alternative will be.

* Altough I wouldn't mind remove that system altogether and adding the buffs to nodes on the passive skill tree. Right now, the Pantheon feels tacked on.