There has been a gender pay gap in professional tennis for a long time, men earning much more than women. It is true that women play the best of 3 and men the best of 5. I've seen many men's matches last 4+ hours, but that's rare for the women. I don't know the exact reason, but I wonder if because men are often physically stronger if that correlates with greater endurance for the long, 5-set matches.
Over the last few years there has been discussion among announcers (who are usually retired professional players) during the 4 major global tournaments, whether male and female players should receive the same compensation. I wasn't aware that the US Open had equal pay, but you do bring up a good argument... Should it be equal pay for equal work?
I don't think the women will be moving to 5 sets any time soon, nor the men to 3. I love your question and glad that someone thought about bringing up the subject.