This is pure speculation on my part. But I've often read that nearsightedness and intelligence/being good in school are correlated. I've also often heard people claim that reading too much causes nearsightedness.
What if it's the other way around? What if some people, like me, are born moderately or severely nearsighted? As an infant/toddler/young child, it might be years before the condition was diagnosed. Meanwhile, activities like throwing/catching balls and climbing trees, which involve gross-motor skills and major physical activity, would be challenging. But reading and fine-motor skills, done close up, would feel much more natural. So nearsighted kids would naturally pay more attention to manageable, nearby things like books and blocks; might become early readers; and might develop a tendency towards the "intellectual/internal" and away from the "adventurous/physical."
Like I said, pure speculation, but it does seem like it might make some sense. Of course, I can also imagine some co-evolution going on (intellectual ability as a compensatory trait to protect us hapless nearsighted organisms).