That's not universally true. I know several people with PhD who have encyclopedic knowledge completely outside their specialisation. Some people are just super intelligent, talented and have enormous memory. The world is not fair.

It's funny but you see the same thing in sports, or I see it specifically in hockey. Phenom kid gets drafted and at 18 has the social skills of the hockey puck he's playing with. By the time he's 36 he's not the player he once was but is a more well rounded individual with age and experience. When you focus all your energy to become the best at something, like a PhD, athlete, musician, whatever, you sacrifice some things along the way for sure.