Well stupid is too harsh and too judgemental, but I would surmise that this comes from an apparent 'weak will' for religious people. To (most?) atheists, religion and god just seem too fairy tale-ish and no hard evidence. It just seems that it "feels" so much better to believe in god and heaven, so why not do it ??? It's so much easier to explain the unexplained through religion, so why not do it ??? Not because its true, but because it makes you feel better about your place in the world. It's not logical, and it doesn't fit into the scientific understanding.
So we think "weak minded" because religious people seem to cling to whatever the bible says, and not think outside the box to see what science can explain (because let me tell you, it explains more and more every single day, and will continue to do so for as long as there exists scientists). It's true that some scientists are religious, but by far the greatest percentage of atheists are those who are well educated in the sciences and understand "how the world works". We can grasp something as complex as life and see the patterns of how chemical reactions can explain virtually everything we know about life. We don't need a fairy tale to explain it.
Can we (mankind) explain everything ? Of course not, nor will we ever, because we are of finite intellect. But science will always advance, and we will gain a greater and greater understanding of the universe around us over time, to contradict dozens/hundreds of religions around the world, just as it has done throughout the history of mankind.