People believe as a residual effect from the practicality of belief for productivity of the tribe. Skepticism in a tribal context is bad. You doubt the leader and you create ruccus. The beauty of it is that god is completely intangible, so there is no immediate negative feedback for having the belief. The belief(s) provides a mechanism for the bonding of the tribe.
Basically the smartest cavedude walked out one day and looked around. "What is this?" He thought... "where did it come from?" So he just started making shit up that seemed to fit his experience. He must have thought "Okay there must be some invisible shit happening that makes stuff do stuff."
Blah blah blah, you get some gods. Since he's the smart one, the people of his tribe look to him for solutions. Now they have some. The tribe continues as such. It turns out that these beliefs define them. As they develop the ideas a bond forms from the endeavor. They were the first explorers into the abstract realm.
Eventually they encounter other tribes who have done the same thing except developed different answers. The beliefs they established in their abstract exploration are for the first time tested, and it turned out they add a cohesion to the group. The shared experience allows them to relate to one another, which gives weight to a preference of whom they associate with (rather than the people they don't know). Turns out this association extends to all walks of life. As trade and politics develops, this fundamental belief in their gods aligns demonstrates their allegiance to other tribe members, preferential treatment and such.
Only in recent years has the general population been educated well enough, and their environment predictable enough that this superficial bond can be questioned without serious risk to the tribe.
So I think in general, people believe in god because people they love have expected it of them and encouraged that belief, all of this out of an ancient basis for power that was inevitable for humans (being capable of abstraction and recall) to undertake. Finally, the cobwebs incurred from this inevitabilty are starting to clear.... but not without much havock.