I agree, it's a farce they only play BO5.
It is a sticky wicket to consider duration in determining the pay. Bo3 or Bo5 doesn't matter. Because if it mattered then non grand slam events should pay same. The point here is that men's tennis is a different sport than women's (Serena's own words). So two separate games played under two separate organizations having their own PnL would result in different pay. Not sure why this is a difficult concept to grasp.