There are some places were it is more overt than others. Engineering, for example, pays women more equally than anyone, though women in academia insist engineering is male-dominated. Environmentalism has far more female representation and pays women even less than the 75% number that gets thrown around.
Lots and lots of reasons are thrown out there but tread cautiously if it comes up. No matter what you say or how you say it, someone will accuse you of mansplaining or being duped by your own white male privilege.
The big reason is often disqualified; women are not only the only ones capable of being mothers, a lot of them like it. When results are normalized for hours worked and family situation, women are actually not paid less.