Because play is wargaming for stressful engagements while also a useful safety valve within hierarchy. Monkeys pull cheeky shit. Wolves pull cheeky shit. Ravens pull cheeky shit - they dive and peck at wolves to see if they can get away with it, and to get wolves used to having them around, and to get wolves' attention so that when they're out in the woods and they start calling raucously it gets the wolves' attention because it means there's something to eat that's too big for the raven to take down itself. You're asking "why do animals play?" and this is a complicated subject but the answer is basically "it's socially useful." If you can play at a rivalry you don't have to exercise it. Two tribes of warriors can peacefully coexist if they occasionally play polo to earn bragging rights instead of slaughtering each other until only one team stands. A group of men in a department can prank the rest of the men in the department as a safety valve so that they can all be friends while one group can still lose.
I heard once that laughing is an evolved behavior that basically communicates "everything is ok between you and me" so you ease into the relationship and the bond strengthens. But I have to really stretch to reach "and monkeys also massively rib to the point of embarrassment and at great potential cost to themselves." I certainly get why it's fun. But why it's so pervasive is just surprising.
The evolutionary advantage of Homo Sapiens over Homo Erectus or Homo Habilus or Homo Neanderthalensis is our ability to use language - sinuses exist for better communication. the sapiens palate is better evolved for communication. Our forebrains are evolved for better communication. The tribal hierarchy practiced by sapiens allows for more sophisticated herd behavior which gave us an efficiency advantage over everyone else. If I haven't recommended this book to you before, I am now correcting that deficiency. Fundamentally, humans don't have the jaws to eat raw and without the ability to cook food we perish... but by evolving the ability to cook food, we were basically able to develop the animal kingdom's first external digestive system. Doing so allowed us to run fast, run lean, and run brutal in ways other animals can not but putting such a vital biological function out in the field instead of inside the ribcage required a communal-insect-level commitment to societal structure. It's only been recently that biologists have started listening to anthropologists: anything that strengthens social bonds is of evolutionary advantage. Full stop.
This breaks the hearts of the old anthropologists. My thesis adviser, on the one occasion that I heard her complain about anything, bemoaned the fact that she was entering into her 70's right as the field has begun to be taken seriously.It's only been recently that biologists have started listening to anthropologists: anything that strengthens social bonds is of evolutionary advantage. Full stop.