Why exactly do you think that a model of gender created by a bunch of people who didn't know anything about germ theory, let alone statistics, describes biological reality? Have you been so unlucky in life that you never ever have considered this?
The modern scientific model is a collaborative effort by many professionals across several fields, refined over several decades. There is no single scientist who is responsible for it. Instead of projecting cis insecurities, it centers trans people and treats us as honest conveyors of our experiences. And it sure is more correct than anything a bunch of illiterate fundies who beat their wives ever dreamed up.