Why don't they make Tennis balls that you can pump up like a basketball? Because they go flat so easily
Swinging the racquet and making contact on the ball where the valve is would give a different flight path to the ball than when striking a more uniform ball.
You can see this in football where expert free-kick takers like Christiano Ronaldo or Andrea Pirlo will position the ball so that he can hit it where the valve is. This will cause the flight path of the ball to bend with a different characteristic.
That's beside from the fact that the valves on balls are typically made from a harder material than the ball itself, so hitting it in this part may cause unpredictable pace, cause the ball to fly long or bounce in a strange way. Where the margins are tight, such differences can mean the difference between winning and losing a point.