Because athiets and both agnostics lack religion.They lack a sense of beleif and a sense of faith,for those that do these feelings don't make sense to them.They prefer to see the world through science rather than through spirituality like christians do.To them,beleiving in god is like beleiveing in Santa Claus.That's why they bash it,it doesn't make sense to them and seems silly so they criticize it.