Over the centuries, religion has engaged in a slow retreat before science. This is true not only of primitive religions, but also for the monotheistic revealed religions such as Christianity. Not all science, of course. Maybe not even most science. Science, however, has demonstrated the the universe is much older and larger than anyone in the ancient world imagined. Science has revealed the spherical shape of the earth. And science has dispelled those religious notions that gods controlled the forces of nature.
Aside from the convictions of young earth creationists, most Christians have accepted the conclusions of modern cosmology, geology, and biology. Granted, they must make some accommodations. For example, Christian intellectuals imaginatively construct such theological theories as the "day-age" theory, the "gap" theory, and the "literary framework" theory to reconcile the biblical account of creation with the findings of science.
It is difficult to make similar accommodations with the biblical story of the creation of mankind. Christians who accept the evolution of the cosmos resist accepting the evolution of mankind. Why is that?
First, the specificity of the biblical story of mankind's creation presents a challenge. It is difficult to tease out any intelligible symbolic interpretation of the passages. Does shaping of the clay into a man symbolize God's shaping of man through eons of evolution? Sounds a bit of a stretch. Second, so many other parts of Christian dogma derived from the creation story. The whole narrative of the creation in God's image, the loss or defacing of God's image through sin, and its recovery through salvation would be lost. If man evolved, where is the sin? Where is the salvation?
Second, the nature of the sciences on which the idea of evolution rests seems to be not as strong as in other sciences. Everyone understands the basics of experimental science, where researchers create a specialized experience in a lab and reach repeatable or falsifiable conclusions. The science behind evolution differs. Biology, of course, utilizes the specialized experiences of the lab, and our understanding of biology provides insight into the structures and functions of living things. The idea of evolution, however, also rests on the findings of archaeology and physical anthropology. These sciences seek to understand a past that cannot be recreated in a lab. In their methods of inquiry, or at least the goals of their inquiry, archaeology and physical resemble history--usually understood as part of the humanities. In fact, the subject of archaeology and physical anthropology once was described as "natural history."
So given the Christian predisposition to adhere to their religious dogma and the nature of the science on which evolution rests, it is understandable by some religious people resist the consensus of modern science regarding the origins of mankind. And if would require a level of curiosity not possessed by the average person to walk into a library and check out a book on evolution. Most people rest content with the science education they received in high school or college. Given the nearly endless variety of pursuits people can choose from to fill in their leisure time, reading a science book probably hold little attraction for anyone--Christian or not.
The alternative--at least from a religious perspective--is worse. This is the blatant rejection of the creation story itself. Liberal Christians adopt the view of atheists and see it simply as a folk tale. And, of course, as might be expected, for many liberals, without the creation and the fall, there is no original sin. Mankind is not a flawed creature in need of redemption.
But they still won't let go of Jesus.