As I've written on in the past, there have in fact been quite a few historical instances of formal recruitment of women. The example I linked is the Soviet Union in WWII, but within the Russian & Eastern European context, there was already some cultural background (both mythical and true) of women fighting in times of need. The mythos of the French Revolution is also replete with claims of women fighting at the barricades, but I am not sure if this is true or not. In non-European societies there are also such accounts, but again, I am not sure of the degree to which these are 'true' or more cultural-mythical accounts. For an example, specifically discussing women in organized warfare in the early modern Japanese context (or really the lack thereof),see this recent insightful answer by /u/parallelpain.

As to 'why not?', the post I linked addresses that in one specific context, but many of the factors carry over to other contexts as well. In a very large number of societies, war has a long masculine tradition, and indeed is heavily embedded in how those societies construct masculinity. I don't want to delve too deep into deep history or evolutionary psychology (the most scientific of all the psychological disciplines ^(/s)) but there are probably very old reasons for this, related to the greater upper-body muscle mass of the male physique and so on, which would have been an important advantage in early hand-to-hand combat. These traditions have carried over strongly into the modern era and continue to heavily shape the social aspect of war. Even in the Soviet example, which reflects a society in extremely dire circumstances, there were debates and hesitation (largely in powerful male circles, but also among women) over the appropriateness of having women fight rather than take on more traditional roles like caring for the wounded, raising children, etc.