I like the way @toly answered, so scientific.
. . .
If the ball weight is light as ping pong ball(6-7 grams), hitting with using human arm will be controllable.
But tennis ball weight is around 60 grams, quite heavy, especially when it travelling 80-100 mph, the momentum will be too much that only human arm + 300-400 gram racket can not control the shot well.
. . .
The faster we swing our arm, the more we loss control in it.
So that why we have to use body and leg to help arm generate racket speed.
When our arm swings slower, our hand can control the racket face better.
1HBH, right-handed, counter-puncher.