One of the studies found that only 5% of the athletes they studied were only children. That presumably means 95% of the athletes had siblings.

Using 1999 data [1], 52% of US families had no children, 20% had one child, 18% had two children, 7% had 3 children, and 3% had 4 or more children.

That means that of children who had siblings in 1999, at least 59% had at least one older sibling, and at most 41% were the oldest sibling. If position within multi-sibling families was independent of chances of becoming a great athlete, we would expect great athletes to be more likely to be younger siblings simply because there are more younger siblings.

If we toss only children athletes in with the oldest child athletes from multi-child families, it doesn't make much difference because with only 5% of athletes being only children there simply aren't enough to tip the balance. It only brings it down to 56% have at least one older sibling, 44% do not have an older sibling.

This suggests that a better question would be "Why are only children less likely to become great athletes?"

[1] https://aspe.hhs.gov/report/trends-well-being-americas-child...