Our ears constantly hear background noises, our eyes constantly see things happening. That may be easy if the brain already knows what kind of events connect with throwing a ball since it will know what to filter out. But at some stage it will not have that knowledge. Obviously, most brains are rather good at spotting links hidden in a 'blooming, buzzing confusion' even before they know what to find. We are born with a special means to track how events link even when they are hidden amongst a great many other unconnected ones.
Autism is due to an inborn disability in tracking.
Now suppose a child is born without this ability. Well, for a start they would never learn that it is fun to throw balls. They may have fun seeing the smile of the face of the person that catches the ball, but they will not associate that smile with them earlier giving the ball a throw. That will stop them being rewarded by interacting with them. Not because they do not enjoy people smiling but that experience will be isolated and unlinked to anything they did.