An atheist probably "believes in" (in some sense) science, yet science is clearly man-made. That is to say, we humans did not create nature as she is, but we do generate hypotheses, formulate plans of study, and develop theories. Light existed before mankind, but there is no reason to believe that Maxwell's Equations existed before mankind -- especially since they are very, very slightly wrong (certainly at the quantum level). But although they are man-made, they are not exactly arbitrary, either; a different human culture or an intelligent species on another planet would almost certainly derive exactly the same Maxwell's Equations (expressed in their own language) at some point. The process by which mature theories are developed, on the other hand, and intermediate approximations along the way are likely to be much more varied. For example, there have been countless scientific papers trying to explain high-temperature superconductivity -- none of them, to my knowledge, quite succeeding -- yet the diversity of conflicting theories does not mean that high-temperature superconductivity does not exist.
Well, theology is much the same. Not all religions are all man-made, but a case could be made that all theology is man-made. After all, theology is also a science. Anyone with the most basic understanding of Christian theology knows that the full reality of the Holy Trinity is beyond the understanding of any finite being; we can make a few basic statements with confidence, but anything beyond them is at best a good, man-made approximation -- sort of how the approximation π ≈ 3.14 is both reasonably accurate and also convenient for our calculations in the decimal system.
And then, of course, there is pseudoscience. Alongside the descriptions of the interior of the earth given by serious geology one can find (especially on the Internet) the "theory" that the earth is hollow. Some claim that Admiral Byrd flew through a hole near the North Pole and briefly entered the hollow earth; others claim that the hollow earth is populated by technologically advanced reptillians, or by humans living in a sort of hippy Shangri-La.
Why do so many people find pseudoscience credible? Ultimately, I think this usually comes back to the "mystery religion" aspect that I wrote about earlier: the Gnostic thrill of feeling that you are one of the elite few to be smart enough (or brave enough) to know (or face) the truth. Much of the diversity in religious belief comes originally from the same Gnostic thrill.
So, by the way, does most of atheism. Someone who simply doesn't believe in God or who doesn't care is not likely to find the question worth arguing about. Most likely he will just play golf (or watch football, or whatever) on Sundays. He may even give lip service to the ambient religion, perhaps to gain approval from his neighbors or as an assertion of ethnic identity. He may occasionally do this as a cultural exercise -- if you watch travel programs, it's not uncommon for the show's travelers in the Himalayas to burn incense to the Hindu god of the mountain, or for visitors to Maya pyramids to obtain the blessing of a local shaman, etc.
That type of unbeliever is unlikely to think of himself as an atheist and is even less likely to proclaim his atheism to the world, except perhaps as the result of a direct question. No; the kind who draws attention to his disbelief does so because he enjoys feeling that he belongs to a unique and highly exclusive club of the bright and brave. In this he is like the sedevacantist Catholic and the neopagan, though they may have little else in common.