Please, don't bring your religious beliefs into this. I don't want anybody to say "because there's a god", because that'll just trigger a flame war. Just think about this: why does religion exist?
I've always thought it's to provide a sense of control; for example, if your people experience a flood, as an atheist you can do nothing. (Yes, I realise you could use sandbags or whatever, but bear with me.) But with religion, you can pray. Humans, as a rule, hate not having control.
So when somebody says "That flood was because of random, heavy rainfall", you resent it and spend your life worrying about more floods. But when somebody says "That flood was because of heavy rainfall brought about by not sacrificing a goat to the Rain God", then you sacrifice a goat and be done with it.
