You still get two-handed control over the plane the arrow takes (before drift) and it’s easy to dial in angle and draw length. A sling bullet leaves tangent to a circle spinning at considerable speed and distance. The fact anyone can hit anything with a sling is a testament to the human brain’s that-looks-about-right capability to treat tools as extended limbs.
The same for the bow, when considering we only figured the archer’s paradox already in the XX century and demonstrated it when high speed cameras were develope.
A sling shoots forward in a straight line and it only depends on the thrower to give it centripetal force.
Slings are still used today as weapons and effectively. And hunting with one, particularly birds, is an extremely complex exercise.
