Your question is a classic anachronism, because it’s absurd to try and understand the world of 3,000 years ago based on the culture of today. It’s like asking why the Bible doesn’t relate to microwaves and cell-phones, and why didn’t Abraham drive or fly to Jerusalem! Thank God, almost everything has changed as mankind has slowly matured over the last thousands, mainly thanks to the gradual moral growth induced by the Bible (the shift from paganism to monotheism, from violent and taking gods to a perfect and giving God, which hastened morality), together with the natural maturation process of trial and error.
The Torah is eternal which means that just as the Bible applies today, it also had to apply and be relevant to the primitive world at the time when it was given. In a primitive patriarchal and agricultural society, where a person is measured by his muscles and the class into which he was born, and women were dependent upon their father’s and their husband’s protection and support, the place of women and girls was considerably different than today, and must be seen in that light. In a society without police protection, especially if a girl’s father was sickly, weak, or died (very common in those days), the only solution was to have a husband protect her. Accordingly, it was totally and unquestionably accepted for girls in all cultures to marry young in those days, and by the way, so did the boys! The Cairo Geniza shows that family life even “just” 1,000 years ago was still relatively similar, where they married young and usually continued living as semi-children in the household of the boy’s family, and so it was in the entire middle-east.
God wisely didn’t force mankind to artificially change overnight (which would have caused rebellion and rejection), but rather took the world by the hand and through many small and acceptable moral obligations towards man, woman and l’havdil, even animals, man slowly but surely was tamed and cultured. Just like the Torah begrudgingly allows eating meat after the flood, even though it’s clear from “in between the lines” that it’s frowned upon, the Torah had to be relevant to where man was at. The Bible advanced us, for example, by obligating painless slaughter, not slaughtering a cow and calf on the same day, covering the blood in embarrassment, and the rabbis added: not blessing on new clothing if made from leather, not blessing a friend on his new clothing if made from leather, etc. Even as the gentiles ridiculed the Bible as “feminine”, extending moral feelings even towards animals, nevertheless, even they eventually “saw the light”.
In addition, the job of the rabbis in each generation is to decree new laws according to the advancement of our morality. About 1,000 years ago, the ashkenazic rabbis felt that man was mature enough to decree prohibitions on taking two wives, and with the aliya of the Yemenite Jews about 60 years ago, the rabbinate instituted these laws for all of Israel, in addition to prohibiting marriage before the age of 16. That (and not the “primitive” days) is the Torah, both written and rabbinic, which should concern you, because that’s where you and mankind are at, at this stage.
You are correct that the difference in lifestyles and the way marriage was treated 3,000 years ago should be pointed out to children when studying the Bible, but from my experience, even the smallest children have no problem understanding that the world has changed (thanks to an eternally relevant Bible)!
With Love of Israel,
Rav Ari Shvat