The wage gap is a really complicated subject that has a lot of mystery and falsities surrounding it. On both sides actually. The ultra-progressive narrative that male patriarchs are signing 25% smaller checks for women is provably a myth. But on the same note, there are absolutely societal factors at play that are more important than 'women don't negotiate for wages enough'.
I subscribe to the theory that the bulk of the reason we don't see as many women CEOs and high-powered businesswomen is because the age where most people pursue an MBA (after a couple of years in the workforce) falls right around the time when women end up getting married and having kids. Because of the way our society treats maternity leave as being 'optional', this derails the careers of lots of women and causes them to fall behind men in the job market.
There are some other issues at play, but I think this is probably what makes the biggest difference. Just my opinion - not sure if any really-good studies have come out and proven it.