The fat content in whole milk or cream creates the characteristic creamy texture of normal ice cream. When you replace milk or cream with non-dairy liquids in nondairy ice cream, the tends to have a higher water-to-fat ratio which means that the ice cream freezes into a much harder block. You'll notice that when you buy store-bought non-dairy ice cream, the instructions often say to leave the ice cream out for at least 10 minutes to thaw before trying to scoop it.