Generally speaking the men do put in more work (best of five vs. best of three) and the men are inarguably superior players ..
I am not swayed by those arguments
It is all about marketability and popularity.
The hard work (if we assume they train equally hard), would then be translated into Usain Bolt being paid less than a 100m female sprinter, because he finishes his heat quicker.
As for training hard. I think the women train as hard as the men. Among women, tennis players train hard, but so do a lot of other athletes, like swimmers. In Denmark the top swimmers are so envious of Caroline, it's off the charts. Every chance they get, they take big swipes at her. One swimmer Jeanette Ottesen (in)famously claimed : "I am bigger than Caroline Wozniacki. She only wins small tournaments". When there's voting among athletes for SPOTY (BT Gold is the local name), the female swimmers vote for each other and awards Caroline 'nil' point.
Anyhoo, I am sure they work hard, but they are effectively on social welfare, as they cannot feed themselves from their chosen profession.
Yes, physically, men are stronger. That's how evolution (or God if you're one of those creationism nutters) has impacted the two genders. Doesn't say anything about entertainment value though, that a man can serve 130mph and women (normally) only about 100mph.
At the end of the day, women's tennis is marketable and popular, so they get equal pay. Women's footballers .... not so much