Because recently the earth was very warm and full of trees. It was advantageous to develop an arboreal lifestyle. But gradually the earth cooled down and the tropics were confined closer to the equator, and the parts of Africa where our ancestors lived also faced a tree scarcity, not all at once but over time. So these primates learned how to walk on the ground again, and the form that took was bipedalism so we could use our forelimbs as manipulators. The equipment we evolved to make jumping through tree branches convenient -- opposable thumbs and rolling shoulders -- is the same equipment that make it possible to throw spears or baseballs.