Mostly because the two genders have different "mating strategies" and look for different qualities in a partner:

- Most women look for a strong, self-assured, masculine and driven man. I.e., a man that's most likely to be a "winner" in life and being able to protect her and her offspring. This type is far from the "nice guy".

- OTOH, most men look for a woman who's loving, affectionate, kind and feminine (other than looking young and fertile). I.e. a woman who could be a good mother for his children. This type is much closer to the "nice girl".

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Our mating choices are mostly based on evolutionary reasons (and they operate unconsciously, not intentionally).

Nature doesn't give a damn about equality, morality or ideology: it's all about reproductive success.