The big part of this answer is, because people don't know the future at all, even one second into the future.

The earth is reasonably stable. Is seems that this and that will happen. But we don't know for sure. If we did, we would always be able to avoid things like car crashes, Fukushima, train and plane crashes, and almost everything else that goes wrong and harms a person. People, therefore, look for something to give them peace. It's religion.