On occasion a question can be answered with absolute certainty, and this is one such. Y'all are wrong. The reason peole does not believe in God is that there is no peole!
Having settled that argument beyond any reasonable doubt, I am officially "hijacking" this thread (in flagrant disregard of the rules) to a wholly different question: why do some people not believe in God?
Like most questions, it has at least one simple answer and a complicated one.
Simple: because God allows it. The whole "freewill" business, y'know, "if God had wanted to make robots..."
For a primer on the complicated answer, I commend a novel to y'all, the only one C.S. Lewis ever wrote (his many other books are not strictly in that style): "Till We Have Faces". Besides being an entertaining read, it deals with the core problem of unbelief very effectively, by retelling the story of Cupid and Psyche from the vantage of Psyche's sister.
A start on the complicated answer:
1- the "some" in "some people" is quite small. Every society throughout history that has ever been recorded, no matter how remote "in darkest Africa" has a profound belief in God(s). In fact, the more primitive the culture, the more strongly these beliefs dominate every day life, AND the more likely they are to believe in a SINGLE great God (like most native american "indians" belief in the "great Spirit of the Sky"). In fact, the very first "specialist" in the society is always the "shaman", the combination priest/physician. Read carefully the writings of some modern atheists like Sartre, and Asimov and you will find they actually believe in God. Sartre recounts an episode in his early childhood of feeling the presence of God looking at him comitting a sin, and becoming angry that He should be so rude, and never again being so disturbed. Asimov in his famous Foundation trilogy winds up with 3 foundations: the physical, the mental and the group that finally dominates all mankind (for the better), the spiritual (in a pantheistic form). There are indeed very few people who secretly do not believe in God.
2- Nonetheless there are some, for so says Scripture: "The fool says in his heart, "There is no God." This is the only passage I know of that clearly speaks of total atheism (please correct me if I'm wrong). As to how these, I'd write further some other time, maybe, that is, should the Fortunes divine it