Allow me to stand your question on its head: Why do you suppose that priestesses were "inconceivable" in contemporary Jewish culture when they were common in other ancient Near Eastern cultures? Why would the people to whom God revealed himself and entrusted with his divine revelation and precepts have had less understanding than their neighbors on this point? Many other aspects of Jewish faith and practice had parallels in ancient Near Eastern cultures: temples, altars, sacrifices, ceremonial washings, ark-vessels, and so forth. Where the religious practices of that day and age were acceptable to God-either because they were grounded in natural law or at least because they were neutral cultural conventions-God allowed them to be incorporated into the religion of his people. Where the culturally accepted practices of that milieu were not allowed to take root in Judaism, we must recognize that God corrected the errors and distortions introduced by fallible human beings.