Draw an imaginary line perpendicular to it from your body center. This line has a fixed length to it. Rotate your body to one side and notice how your imaginary line arcs inward from the monitor. As a tennis player you face the reality of a ball angling away from you. If you rotate your body during the swing, this means both your racket and your body are moving away from the ball at the same time the ball is moving away from you.
Linear momentum is an easier and more reliable source of power than angular momentum.
Its mathematical equation is simpler as well. When a tennis player rotates, it's overkill, counterproductive, and everything gets more complicated. They rotate more, and their accuracy suffers. The length to the linear shifting of your body weight is small. The tennis balls placed below the center of my body in photo 4D represent this length, and the arrow shows the direction of the shift.