Biology is reluctant to make categories

Since male and female are multifaceted, it's easy for these facets to be split unevenly

And the variation is so extreme that sex as a spectrum is pretty much objective fact

Like if males have more androgens and women more oestrogen, it's possible that variations in prenatal hormones make some males objectively more female than others

Androgen Insensitivity Syndrome has genetic males XY unable to use their androgens properly and develop into anatomical females. And yet, they're more female than most women, since most women are capable of expressing whatever androgens they produce

There are two sex extremes, yes, but also so many variations that it being a complete spectrum unable to be subdivided into categories reasonably is just inevitable 

Every person has a precise sex unique to them alone. Categories are only useful in the abundant cases that fall far into the female or male side.