Why Don't the Women play 5 sets in Grand Slam Tennis?

I'm just sat watching the tennis. The French Open, Women's Singles Final for 2015 and, as usual, I am wondering why the women get away with only playing 'Best of 3 Sets'. The Men's Final is 'Best of 5 Sets' so why the inequality? Surely, in times where equality is pushed with such fervour (rightly so), it is only right that the women get the same prize money for the