I know tennis balls go flat (even if all they do is sit there) because the interior of the ball is actually pressurized above normal atmospheric pressure, and over time some of the air leaks out.
The question is, why aren't the balls manufactured with some material lining the interior that's properly airtight (creating balls that should never go flat)? Do the rules of the game prohibit that sort of thing? Or is it more that manufacturers would rather keep selling you new balls? Or something else?