In large parts it’s because of the choice of employment and losing out on work experience when staying home.
It should be socially accepted to work what the hell you want. But I don’t see any way to codify a fair gender ratio into law. So this is a really tough nut to crack, because it requires a mind change by all of society.
Arguing for equal compensation regardless of experience makes no sense. Unfortunately only one parent is equipped with the hardware to make and raise kids. So we need a way to enable more equal distribution of work. But if women are just naturally more inclined to care for kids (which would make a lot of sense from an evolutionary perspective), then this gap will persist unless we force people by law to work or take care of kids more.