Why believe in God?
Well, at least some people realise life can be really shit - and even those that are successful or win the lottery end up dead. So, the idea that some kind of afterlife with God awaits the believer makes them feel warm and fuzzy inside.
Also, in the real world, good things happen to bad people and bad things happen to good people, and that sucks.
Start believing in God, however, and the robbers who made off with all your possessions but weren't convicted by a human judge can now be punished by God in the end.
Of course, my answer doesn't cover all the reasons why people believe - some are brainwashed into it as children, some believe in God because they're scared non-belief makes him angry, and some just do it because it's what their family or tribe has always done.