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.
Best Answer
Your question implies that football properly describes the game and, by extension, the ball itself. I wasn't sure, so I checked and it is indeed the case historically. Etymonline says:
So, usage of the term to refer to the ball instead of the game is quite old, but it still took more than a century after the name of the game was established. Nothing is said of the reasons we don't say “football ball”, so I assume that it was the obvious: to avoid unwarranted repetition.