CRobinson2038:
I suspect it's because of the way the game program works.
As it is now the game program is such that there can be only 2 ball performance profiles available in the game - the ball the player equips the bag with before the game starts and the generic starter ball profile that is the same for every game.
Imagine the difference in the complexity of the programming if players could equip their bag with a variety of ball makes/models.