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.