Just trying to put this in the most bare terms, there's obviously plenty of edge cases and caveats around gender, but:

a) Society places a greater burden on women than men for child-rearing. b) Women who take leave lose out on career opportunities. c) Men receive greater opportunities to advance their careers relative to women, since they are expected to take less leave.

Even in the presence of equal parental leave, men (or just as often, their employers) can lean on the social expectation to return to work sooner, disadvantaging women.

This becomes a bit of a chicken-and-egg problem since the culture and structure are mutually reinforcing. From a policy perspective, structure is easier to change than culture. In my view, that makes it a logical first step.