Because they weren't taught in Florida, or by people following that teaching. Girls were taught to grunt on hitting the ball. Boys weren't. You don't hear Nadal or Federer or any American man doing it, yet they hit the ball far harder and with just as much effort
The most revealing moment was watching Sharapova in a US Open match either last year or the year before. She was losing, or doing badly, and just stopped screaming. It was surreal. The commentators picked up on it, and all.
That's enough for me to believe that it's a choice. And, therefore, nothing more than gamesmanship.
No, I was at Newmarket races. Even the fillies weren't screaming or grunting there ! Nadal has been out of the game for a while. Maybe he's feeling the effort now he's back.
I watched the Sharapova-Azarenka match with the sound off - I knew what was coming. Never heard Nadal make so much noise during a match as he did to-day, but a wonderful semi-final.