Because it's a way of manipulating and controlling the masses. If you are told if you don't behave in a certain way or subscribe to a particular belief system then you will endure an eternity of agony, people fall into line. Personally, I have a mind of my own and can see those religions for what they are and God as being pure love and light. The only Hell here is in living a half life full of fear, then dying and being so terrified of Hell that they won't cross over into the light.