Because they think that if you are childless, that is god's will, and IVF is using evil science to circumvent his will. You've got to remember, these are people that believe that understanding and manipulating the world through science is inherently evil, until of course they need cancer treatment.
Even if the open windows of science at first make us shiver after the cozy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own - Bertrand Russell