Bloody Hotdogs! My answer to your question is: Because there's an undeniable tendency in humans which is stronger in some more than others for what some people term "religiosity". Other's call it "spirituality", which encompases the feeling that there's more to the universe, including a meaningful and ultimate purpose and a higher intelligence. That is the force that has caused every culture since the dawn of time to have a concept of "god", something that is very difficult to explain in Evolutionary terms. I came late to this discussion, so forgive me is someone has already addressed this topic with my same comments.
Perhaps it's the misguided need that spirituality can create that makes people congregate into religious groups. After believing for decades and then losing all my preconceived notions about God, I have come to respect and appreciate the need some individuals have to be spiritual. I recognize it in myself, although I have reasoned that I don't need the trappings of rite and fanciful imigery to accomplish the same thing.
At the same time, while believing in the material world above all, I realize that Atheism cannot answer all the questions about what is or isn't. That does not force me to make an alternate conclusion. That leaves me in a difficult place, but one I've learned to be comfortable with, a place from where I can sincerely say: "I don't know". The great thing about my "not knowing" is that it continues to push me to ask more and more profound questions rather than sit complacently on the sidelines. I walk a tight rope every day.