[philwelch]

It's not "the best players" who struggle with free throws so much as the taller players. That's probably because if you're seven feet tall, it's a lot easier for you to become a valuable player at the NBA level than if you're six feet tall, so guards tend to have more developed skills than centers. Though even this is starting to change in recent years.

[nradov]

I've seen a physics analysis which indicates that being very tall is actually a disadvantage for shooting free throws because the ball approaches the hoop at a shallower angle. Essentially they're aiming for a smaller target. Shorter players have to throw the ball up in a higher arc.

[philwelch]

That's true to some extent. Hand size can also make it more difficult to hit shots. But then again, there are plenty of examples of extremely tall players or players with extremely large hands who can still hit free throws reliably.

The main difference is probably that if you're seven feet tall and athletic, you have ways of providing value on the basketball court that outweigh being a liability from the free throw line. Not so much if you're much smaller than that.