Mechanically speaking, launching a projectile is one of the first things we figured out how to do. The sling and the atlatl*, the first two range extenders we figured out, are extensions of our basic throwing ability. They draw on the same proprioception and intuition as just throwing with an arm.

* Because atlatls had fallen out of use in Europe for literally thousands of years, any words we might have had for the devices disappeared before our earliest attested recorded language vanished. So our English word comes from Nahuatl.

Tangentially related: I love busting out "atlatl" in a Scrabble game.