Why don't atheists believe in God if atheists admit that there is no proof that He doesn't exist? That's like asking why adults don't believe in the tooth fairy simply because there is no proof that she doesn't exist. But more to the point, compelling evidence for the existence of God is sorely lacking. On the other hand, there is ample evidence that the reasons we humans have believed in God throughout history are not good ones by today's standards. Further, there is ample evidence that what major religions claim about God is patently false.
Paleoanthropologists trace the belief in God from its beginning as animism. Animism, the belief that inanimate objects have a spirit, in turn gave rise to various pantheons, like the pantheons of the early Canaanites and Sumerians. These finally gave rise to the various creator gods like Chronos or Ptah. Throughout history, as the concept of God evolved, we humans used the gods to explain natural phenomena like stars and thunder that we could not explain in any other way. As we increasingly developed a better understanding of our universe, there was increasingly less need for divine explanations. For example, we once thought that the sun was pulled across the sky by a god; now we know that the earth rotates about the sun. In the same way, theists today believe that the universe was created by a god, even though there are natural explanations. So, to finally answer the question, if we humans believe in gods for all the wrong reasons, and there is no compelling evidence that gods exist, then it is unreasonable to believe that gods exist.
