Many of the "religious stories" come from thousands of years ago. At that time scientific understanding was at a primacy. Even the Greeks while developing philosophy developed some quite "unbelievable" philosophical explanations if they are taken literally. Thus people who were not as advanced as the Greeks scientifically or philosophically had even much greater "incredible" stories to tell. In other words, the cultural level of the society was a factor in the creation of the type of stories it would create and the written form they would give them.
I place the word unbelievable in between " " because what one person considers unbelievable might be a tenet of faith of another. So I think the use of that word already shows some bias against "religious stories" in general. However, I don't disagree with the statement as such because I think that the stories have been made "unbelievable by the manner in which people of faith, or of blind faith, have decided to interpret them. As I said above even more then rational or philosophical explanations of the origin of the universe by the Greeks had its limits and cannot be taken to literally, Literal interpretation of ancient scriptures, in fact, also contribute to making the stories "unbelievable".
The story of Adam and Eve for instance was not intended to be taken literally as meaning that chaos and evil came upon the world through a simple disobedience to a commandment not to eat of a literal fruit of a literal tree. The Hebrew Scriptures, as well as the Christian one, use many symbols to refer to regular things. But people insist in using a literal interpretation because it is easier even though it leads to some irrational conclusions with poor basis on the very text they intend to interpret.