I'm an averagely short, fitter than average 40yo woman.
In a world of male-dominated design, not a lot fits me, if I go to an event, I slip through the neckhole of hi-viz vests because I'm not a male unisex size small. Going backpacking, it was always a struggle to get a rucksack small enough to fit, but big enough to carry what I need which is only marginally smaller than the volume of DH's gear as it's only a marginal difference on clothing volume smaller. Many larger vehicles are harder and tiring to drive because they're not designed to be driven by people with size 2 feet and a 26" inside leg. When things don't fit in a war zone, you're less efficient, slower, tired, less protected (bullet proof vest rather than hi-viz) and that means you'll take more down before you go down.
Males are more consistent in biology. Their hormones are stable (no periods/ pains/ PMS/ peri-menopause) They're easier to allocate equipment too without those awkward breasts/waists/hips. Then there's birth injuries which can lead to permanent difficulties
In an emergency conscription situation, it's much simpler to equip and train men with more efficient outcomes.
We're also once again underestimating the importance of caring.
If one parent had to be conscripted and the other care for our DCs, I'd be the better carer. DH is a loving father and could do it if he had to, but given a choice the DCs gravitate to me for comfort. Maybe it's that due to umpteen circumstances I've ended up as the SAHM, but also from the very first feed, I could give them the comfort of milk in their first 1-2 years, and again biology can't be ignored. The first bond in infancy is usually the mother and that predates birth.
In peace time there can be reasonable adjustments to even up outcomes for women to men. There isn't the time to work that out in a sudden invasion.
While I'm fitter than DH and can run half-marathons, he would be better in the military. As a male he'd gain greater strength and speed and do it faster than me. Stand at the finish line at parkrun and it's predominantly men finishing faster than 22mins. The sexes don't become balanced until closer to 30mins and females dominate the slower participants. My stamina's good, but that's little use if I was shot because my sprint is limited.
The two sexes are not identical.
The patriachy also favours the logistics of catering for men in military/ combat roles.
It's more practical to conscript men.
Don't underestimate the importance of caring roles and the dangers faced by civilians and refugees.