There are 3 factors which determine the value of an investment.
Time invested
Amount invested
Return received

Women are on the back foot with all three.

Time
Women live longer
In the 1800s women lived to age 33 and men to 31.
Today women live to 83 years and men to 79 years. In general, women outlive men by about 5%.
So, women need to plan their retirement funding more carefully. Living longer means that more provisions need to be made for those extra years.

Amount
Women earn less
Traditionally women have been left behind in the workplace. Key positions have often been taken by males. I guess the reasoning has been that women break their employment when they have children. Earning less translates into saving less. Until women are on the same financial footing as their male counterparts their ability to invest more will always be a struggle.

Return
Women are more cautious
Women tend to save for a shorter time as they focus on the day-to-day needs of the family. This hinders the effect that compounding has on investing. Saving in a cash account will not yield the returns found in equities over time.

Times are changing
There is strong evidence of women making progress when you see women like Christine Lagarde heading the IMF, Teresa May past Prime Minister of the UK, Angela Merkel leading Germany, Jacinda Ardern prime minister of New Zealand and recently the first woman to be elected as president of European Commission, Ursula von der Leyen.