Because your biology doesn't determine your destiny within society and the interests of liberty is not in tying you to it. For instance; a woman is entitled to choose not to have children, for any reason or for no reason at all. While it is a biological fact that only women can have children (or indeed can breastfeed, etc.), it does not follow, as a fact about how we conduct our society, that women are uniquely capable of raising children, or that it is their role within our society to birth & raise children. When we structure our society in an essentialising way, we cause create injustices for both men and women (to say nothing of trans and nonbinary people, who are erased by these essentialized framings, which for instance can lead to them being discriminated against in the adoption process), though women bare the vast majority of them.