Temporarily. On a local level.
It has to do with the setting's origin mythos.
Before the beginning, there were Powers. Angels, gods, makers, whatever you want to call them. They were themselves made by some Higher Being, but He's pretty hand off.
The Powers created the universes, designed the physical laws, made elements and people and basically everything.
Except for one Power, who stood aside and watched.
The Lone Power wanted to contribute something special, something that would leave His mark indelibly on all that is.
So just as the Powers finished fashioning existence and were about to set it in motion... The Lone Power stepped up and created entropy. He invented death.
The Powers couldn't remove His gift without invalidating all of theirs, but they cobbled together the Laws of Thermodynamics as a patch on reality to compensate. And they removed death's sting by creating wizards.
The purpose of wizards is to mitigate death, to slow down entropy and make its effects less painful.
So now the Lone Power goes from race to race and individual to individual, peddling his fear: he fights wizards to make entropy scary and to increase it.
Wizards turn out lights when they leave a room, convince a tree to stop strangling its neighbor's roots, talk a sun out of going nova before its time, comfort a grieving child who has lost his mother.