You do raise a fair point by bringing up cure 3, gravity, and toxikon. But having holy be targetable on enemies would make it pretty strong since it has a huge range and stuns. Personally I prefer it being centered around the user as it looks visually more pleasing.
I believe the main reason the devs made holy only cast from your character is a design choice. They probably want Holy to be an aura emitting ability and not a projectile/bomb that you fling onto your enemies like toxikon and gravity and the cast animations reflect that.