Women were functionally disabled by having children, spending a significant amount of time either pregnant, or breastfeeding. This makes them the natural parent to focus on raising children. Also, in nature, losing 1 parent has a relatively minor drop in survival chances compared to losing 2.
This ends up with men being more “disposable” than women. If 1 group needs to flee with the children, while the other holds off an attack, it’s most sensible for the men to defend. The women would provide a final line of defence.
This also massively effects the risk/reward balance. Ultimately, a woman’s ability to have children is limited by her biology. The limit on men is FAR higher.
For women, once they hit the resource requirements to support 2 dozen children, there was relatively little real gain. A successful man could (in theory) have hundreds of children. Genghis khan being the most egregious example. Taking large risks for large gains makes sense for men, in a way that just doesn’t for women.