If you hit or catch 1000s of balls the player will figure it out, it is not that hard when you do it all the time. I agree that good instruction is important but I will take reps over instruction.
There is no substitute for doing that's true. But like that car commersical where the father is showing the kid all the wrong things to do with throwing a a ball and saying - good job - that kid can practice all he wants that way, he's just going to get really good at being really bad.
If a kid is OK at a skill I agree they probably will figure it out by doing it over and over but if they've got a flaw it's going to get magnified if not corrected early and be very difficult to correct later after they've ingrained it with 1000s of reps.