Archeology disagrees with you on that point. Ancient people tended to switch gods and religions fairly regularly, depending on who conquered them or the ease of doing so. How many times did the Jewish people abandon their religion? Religious fanatacism is relatively recent.
Ancient religions offered moral tales to teach one how to live life, to tell what to expect after death and to maintain their own cultural values. When those values changed, so did their religion. No one has held onto a religious dogma in ancient times nearly as long as we have held onto some of our modern religions. Ancient Egyptians, for example, believed their religion offered spiritual truths, not worldly truths. They knew that the world was a globe, for example, but that is not the religious picture of the world they accepted - and that dual acceptance of things was fine for them because they understood a spiritual truth is not the same as a worldly truth.
Ancient religions also did not dispute or fight against new worldly findings like the Catholic Church used to do. Christianity takes archeological discoveries as an affront sometimes - the ancients would not have been bothered. Most of their religions would have adapted to a new world view. They were used to that. Few ancient religions had a concrete, unchanging "book" like modern religions do (Christianity and Islam, mostly) that has to be adhered to.
I don't think ancients were as "immune" to symbolism as you imply. Even Jesus is said to have taught via parable, which is not unlike teaching via myth... the ancients expected religious material to be taught in such ways. No one (that I know of) believes Jesus was telling the literal truth with those parables, but that he was relaying a lesson. Thus were the ancient myths taught - as non-literal lessons.
Sure, they believed in their gods but not in the literal truths of their myths. I have had reason to research mystery religions lately. Even modern ones, such as Scientology, do teach different "truths" as they move up in the religion. The ancient religion of Mithras, for example, expected everyone to go through the first four ranks of their religion, but only the clergy went through the following three. Ancient Eygptian and ancient Greek religions also worked this way. Even early Christianity seemed to work this way (Gnostics, for example).
I also don't think we are as immune to symbolism as you imply. There are symbolic archetypes that appeal to us, and continually reoccur in literature, movies, art and even in advertising. Ever notice how it often rains in movies when a major change is happening? Water is symbolic of change. Even the winds ripping through Vader's cape when he tells Luke the "big secret" is symbolic. Of course, Luke's fall after the revelation is an obvious fall from innocence for Luke. Looking at Ralph McQuarrie's concept painting of Luke right before that fall, His arms are outstretched and his feet together on the little ledge - looking a lot like a familiar sacrifice... Anyway, even the cutting off of Luke's hand and Vader's hand later, can be emblematic of the dismemberment of the gods, a common theme in ancient religions. Luke and Leia have a very Osiris-Isis type of relationship.
I think religion is like Frankenstein or the Star Wars trilogy. They do offer something on the surface but can be a whole lot richer, if less mysterious, when one delves into them. I am sure, however, that there are people who will insist there is nothing to Frankenstein or the Star Wars trilogy beyond the surface presentation of the stories - just like there are religious people who claim the same about their religion. I have heard religious people (Catholics, mind you) who have denounced the LotR as evil and Satanic, despite the subtle Catholic allegories and symbols throughout the tale.
I once read that C.S. Lewis once charged that Tolkien was too subtle with his symbolism and allegories and Tolkien responded that the imagery he used is more powerful than the beat-them-over-the-head-with-it approach Lewis used with his much more obvious allegories. All I know is that when I was in the 7th grade, LotR resonated with me, while "Lion, Witch and Wardrobe" made me roll my eyes at the obvious retelling of the Christ story, a story I was already bored with - and I knew almost nothing of symbolism in the 7th grade. Was I immune? No. Even if we do not consciously grasp the symbols, I think they do work and that we are affected by them.
Good point about cognative dissonance. I teach that concept in Strategies for Success at IBC. 3 points for using scientific terminology! I think the success of Star Wars is less about special effects and more about symbolism and myth. Perhaps the reason so many people turn to religion instead of science is because the symbolism of religion resonates stronger within them than the cold logic of science.