Maybe it's the other way round - maybe it's the women who are "normally" ambitious, but it's the men who feel more pressure to be successful and thus ambitious

Maybe it's the other way round - maybe it's the women who are "normally" ambitious, but it's the men who feel more pressure to be successful and thus ambitious (because women seem to value that more than men, in average, as far as sexual relationships are concerned), so you see more ambitious men than women. I'm guessing both effects have an effect in reality.