It is not surprising why people believe in a god and follow some spiritual book. It isn't that difficult to understand. People fear death. That is the simple explanation. If you take time to ponder on being Mortal it can be quite frightening. To KNOW you will die and it is forever is hard for many to accept. To accept means there is no justice and life is not fair. A belief in something more than a finite existence gives comfort to many.
The idea of seeing a child in another life after losing your child to a horrible disease, is more appealing than accepting that nature is both cruel and beautiful. Evolution is an important breakthrough in science with the transitional fossils being found to help answer our origins, but it doesn't answer the questions people have.
Why? and What? Why are we here? What is the meaning of life? In the science world these question are meaningless but to the average person they mean everything.
Then there are many who see how complicated life is and how so many things have to be in place for life to exist, and they just can't accept life is done without intelligence. Science can explain the transformation of life but the very beginning they still have not answered. The planet has only one Gensis and life is not spontaneous being created now. The study of abiogenesis is making great strives to answer these questions but scientists still have been unable to demonstrate life generation from non-living matter. Spiritual books answer these questions and offers simple explanations.