Because it's a polite fiction that men and women are equally capable and expendable at warfare. Successful and enduring human societies practice traditions around warfare that reflect this.
It's more that women are the less expendable gender. If you send the women to die on the front lines, who is going to birth the next generation to replenish your population?
I mean, the mongols were probably the most successful military, taking on armies five times their size with better metalworks. They used women as fighters. Japanese Bushi ('samurai' class) also had women in their ranks, and Celtic traditions had women not only serving in the army, but very often as arms teachers (military instructor), and sometimes war leaders. Some of the army leaders who troubled Rome the most were women. You can also take a look at the Vikings if you want a fourth example.
The fact is, western military traditions it sexist for no good reason. Yes, the strongest woman will be weaker than the strongest man. Yes, it you take a sport like swordfighting, the best woman will be at the level of the 50th best man. But we're not talking about taking champions on a 1 on 1 duel here. We aren't even talking about fighting. What really matters in armies is endurance, and women are close enough to men on that that it shouldn't really matter.
And even if you want to think of war as a succession of duels, war have changed in the last century. Women are just better as shooting than men, especially when standing.
In fact, women tend to outdo men in extreme endurance competition. It’s part of the trade-off for a lower ceiling on absolute strength.
But yes, in modern warfare there are as many jobs if not more that require precision or some level of intelligence as those that require brute strength. Even if fewer of them are right at the front line, they’re just as important.
You don’t send a large percentage of women to the front line in wars of attrition because their deaths mean a greater loss of future reproductive capacity than men’s do. But the way countries like the US have waged war over the last 75 years (with a relatively small surface area of soldiers put directly at risk), that’s less of a consideration. The counterpoint might be a border war without massive air superiority on either side like the current one in Ukraine.
Not just endurance sports. Anecdotal evidence, but female rock climbers have been more skilled in my experience. Perhaps because the brute force escape hatch isn't as available.
Rock climbing is pretty weird. At the top level, it is clear male athletes have most advantages: height, and shoulder strength, that make routes designed for male very hard to compete on for most female athlete.
On the other hand, routes set for female athlete are also very hard for males to compete on. Some of them you can bruteforce (with strenght a la Janja, or with size), but some of them male joints just can't handle the rotations needed.
Still, a male athlete would do better on female route than a female athlete would on male routes imho (at the top level, at mine it just doesn't matter).
But when you're rock climbing in montains with people who have the same experience, yeah, women tends to do better. And even when you're the most experienced, women tends to get better faster especially when you're doing "multi-pitch climbing" (google translate on this, i hope that's right) for a few days in a row.
