Capable at what? In war fighting context is very important. Some combat operations are better left to strong men.

It's not about capability, it's about value.

Women's lives are valuable, men's are not. This has been the case across basically all societies in human history. Losing a ton of men really doesn't matter too much - especially young, family-less men.

Losing a lot of women, though, is really really bad.

One woman can have one child a year. One man can have hundreds of children a year.

This isn't sexist, it's reality. It's the rationale for male conscription and male disposability.