There were female rulers with notable power and influence (Eleanor of Aquitaine, the Empress Maude and, if you extend to the post-medieval period, Elizabeth II and Isabella of Spain, or back to the Roman period, Boudica) and some female soldiers (Joan of Arc being the obvious example, but there are suggestions that women fought in battles even as mercenaries before that point). It varied by country and by case, but often women were also left in administrative roles over castles and towns whilst their husbands/fathers/brothers were off at war. Women's education, for the most part, was generally not noticeably worse than that for men. And of course no-one had any health care, though there were contraceptive methods (of dubious reliability) around as well.

I think the point also is that GRRM has researched the real medieval period extensively to write ASoIaF and the role of women is part of that, as much as food or warfare or religion or politics. The point isn't even that GRRM has downplayed the role of women compared to real history (I don't think he has at all, and in fact between Brienne, Daenerys, Cersei, Margaery, Arianne, Asha and Catelyn around at once he has a lot more notable and powerful women around at once than at almost any single time in the real medieval period), but that the reasons for why women in general had less power and influence in the real medieval period are, in many areas, not comparable in Westeros.

The biggest change in the mindset of the medieval period and today is that religion was not optional. The Bible was taken as the absolute truth of all things and the notion it was incorrect or wrong was utterly unfathomable (resulting in the incandescent rage that was caused when people did start questioning and trying to change it, like Martin Luther or Henry VIII). Eve being responsible for the Fall and the Original Sin wasn't mythology, it was taken as incontrovertible fact. That had a strong impact on the view of women in the time period.

For ASoIaF the same explanation does not exist. The Seven are a more equal and enlightened religion with (as far as we know) no comparable negative stories/myths about women. As a result we are left with the impression that the sole reason women in Westeros are less powerful than men is down to martial prowess alone, which seems overly simplistic.