Because it is their religious beliefs.
I am not religious and I believe in God. I believe in God because someone with an intelligent mind had to create the universe. The universe is more than just a mere "big bang."
People believe in God because of the fact that they look to a higher power to answer all of the questions that baffle them. They feel that if a God is in existence then it must be the power that makes everything and makes everything happen, instead of explaining it scientifically or due to the lack of scientific explanation. Also it can give moral to the people due to the fact that they can look forward to something with the pending of inevitable death.