That gap also exists in the middle-school and high-school levels [0] (at least in the US), so it fully reduces to the simpler question of why that's true. We should be able to agree that by the time we're discussing professional mathematics there's no expectation that it would be randomly distributed among genders.

[0] https://economics.mit.edu/files/7598 (admittedly a bit old; maybe a lot has changed in ~10 years?)