Because if Avram understood the incontrovertible logic of huju, he would unequivocally agree. Therefore, it is per force that he simply didn’t understand it.
Huju doesn’t like people who learn more or keep mitzvos better than he does. With a paltry education, how exact can one be in shmiras shabbos before putting up one’s hands in frustration at the son/daughter who comes home from Israel with a black hat/acceptable tznius clothing, pointing out error upon error, questioning their ability to eat in their parents’ home?