Because the watermelon has a mind of it's own so therefore it grows that way. Not you silly person they just grow that way it's their natural shape and also did you know you can grow a square watermelon?
I don't know if they're still doing it but years ago, in Japan they were growing square watermelons. They had these crates (squares), and when the watermelons were very small they put the still growing watermelons in the crates, on the ground at the plant. As the melons grew they took the shape of the crates . It was odd looking but certainly easier to stack or jeep in the fridge.