Because its a woman game, bro...they don't possess the man's stroking power, just few of them are able to hit winning forehands (Williams sisters, Masha, Ana, Safina). Most are just returning the ball and chasing all the balls awaiting the inevitable error from opponent
I highly doubt you could defend anywhere near as good as JJ, and no, she isn't a pusher.
Okay, but she doesn't do anything that a long distance runner or sprinter with a little hand eye coordination can't do. There have been at least 10 points where she just puts a racquet on the ball and runs down the next shot until Lena hits an error.