Years ago, when I was in the service, I had a roommate who was a Sikh from India. One year, he went home to visit and for his sister's wedding. When he came back, he told me he was in town and was walking by a Temple where some old men were sitting outside and smoking Temple Incense. He said it was daytime, but it was one of those days where you could see the moon. He said he pointed up at the moon and told those old guys that Americans had "walked" on the moon. He said they burst out in riotous laughter to the point of choking. So he asked them what was so funny. One of them said, "How can someone walk on a God?" and they burst out laughing all over again.
Now we know for sure the moon is a big ball of rock. Americans have been there more than once. Even Christians and Muslims and Jews know the moon isn't a "God", but it's a big rock. However, those old men still believe in spite of what anyone says. Do atheists see any difference between Christians, Muslims, Jews and those old men who believe God is a big floating rock?
When young (and still stupid heh) I gave more credibility and legitimacy to a religion based on its age. Growing up I regarded Catholicism as more reliable than the Protestant versions for example, though ultimately believed Judaism more than them being Jewish. Once I began studying religions other than my own though I came to realize both mine and the Christian faiths were relatively new and much older religions existed like the Greek-Roman deities and older still Hinduism.
I think first and foremost, people believe everything their parents tell them as with Santa Claus. As they get older and get more general knowledge they come to realize a lot of things were lies. Motive aside, eventually we notice how pretty much everything was a lie, or is a lie. But these lies aren't necessarily negative or malicious. They paint a picture that good and evil exist as does ultimate responsibility (don't be evil or else...) And to the mind of a child, having things simplified isn't such a bad idea.
For adults then, those who truly believe in gods absent any proof of them must have the minds of children. Either by avoiding advanced education, or having some clinical retardation. Though most only say they believe for any number of reasons, usually self-serving.