Very simple: People are dumb, people believe what their parents teach them, and pass it onto their kids... the answer is: They don't think outside their little bubble.
Remember its not the truthfulness or what kind of religion that matters, all that matters is if a religion promises a life of plenty / happyness or afterlife as a reward, all the major religions share the "promise / reward" strategy that keeps them alive. If the biblical god promised them a bag of chips instead of eternal life, do you think christianity would have lasted that long? hah.
Many humans can't accept the finality and utter destruction of their friends and family at death, on top of that, before modern times NO culture knew the hitsory of the world, so they had to make up common stories/myths in order to bind society together.. and so religion is an evolutionary coping mechanism that for most intents and purposes has just outlived its usefulness.