I wanted to know why there is a hole in the ball (basketball, volleyball, handball) to fill the ball with air. Why can't the ball come with filled in air and fully sealed so that there is no loss of air?
Large inflatable balls such as soccerballs, footballs and basketballs have an internal rubber bladder which needs to be inserted by hand into the carcass of the ball and inflated to the desired pressure to suit the user (eg: basketballs can be inflated to the produce the desired bounce height for the individual user).
Also, transporting deflated balls from the factory to distributors who package & pass onto retailers saves shipping volume & hence cost.
Small balls such as tennis balls can be pressurized in the factory, but will lose pressure within a few onto or so after being opened (they come in a pressurized can).
Also, the higher curvature of small balls allows them to 'spring back' more easily when they bounce, even when they are not pressurized, although unpressurized balls don't bounce as high as inflated balls.
The ball will slowly leak air because it's skin isn't perfectly impermeable to air molecules. Unless there is some way to refill the ball it will slowly deflate.