Because Hashem created men and women differently and he gave each their own set of mitzvos pertaining to their gender as well as mitzvos for ALL to follow. The mitzvos he gave each gender pertain to their particular weaknesses and strengths and what HE made them capable of. Why do women bare the children and the men never have to feel the pain of childbirth? Because of Chava’s sin. With that comes a whole set of mitzvos pertaining only to women. Men have the mitzvo of bris and that is obviously something women can’t do.
Judaism did not separate the genders Hashem did, and the laws he gave us are designed to serve HIM to the best of OUR abilities.