We know that you need a ball to play cricket, golf, or tennis, and we refer to the balls used in those sports as "cricket ball", "golf ball" and "tennis ball" respectively: you take the name of the sport and then add the word "ball".
But as "football" is the name of the sport itself, why isn't a football referred to as a "football ball"? The same applies to "netball ball", "basketball ball" and "volleyball ball" of course.

