It's a game, it's called 'try and catch me'. Understand what playing is? Dogs don't have a sense of humor and they pretty much function by instinct. The real reason for chasing a moving object is to hunt it down, kill it and eat it.
Since dogs have been domesticated their behaviors have been modified, they have been taught not to bite or chew their humans, no jumping up and knocking them down and because they are missing the alpha leader of a dog pack to give them structure they turn to their human to teach them, make boundaries and they follow them instead.
In the wild it's not natural for dogs to chase balls, disks or sticks but the hunting instinct is strong in a dog and so is the will to please the leader so a combination of instinct and obedience gives them the desire to play with us weather it's a game of catch and retrieve or catch me if you can.