It’s more than just the air that gets lost from a ball, the felt is delicate and even small changes in distribution affect the spin. The stresses in tennis are

It’s more than just the air that gets lost from a ball, the felt is delicate and even small changes in distribution affect the spin. The stresses in tennis are intense, with 100 mph common for even pretty low level amateur serves. Most players keep used balls after a match for practice hitting too, and they stay usable for months. My aunt has been playing competition tennis for about 50 years now, and she jokes that even with all the inflation on other stuff, a can of balls cost $3 in 1970 just like it does now, so they have definitely been making some pretty big efficiency gains in the manufacturing.