One reason is because your chromosomes don’t control genital development, your hormones do. So if you’re born with XY chromosomes and your testosterone receptors don’t work then you’ll develop female genitals and a generally female physiology (minus reproductive organs).

This is all separate from gender expression obviously, but things are hard because the world is complex. If you haven’t seen or experienced this complexity in your life, that’s fine. But don’t diminish the complexity of other’s experiences just because they don’t match your own.