I think that the reason people aren't so touchy about athletic prowess is that it's easy to rationalize away any differences without affecting one's feelings about one's child. Intelligence is seen as static by most people, whereas athletic prowess is always in large part the result of tremendous amounts of physical labor.
I'll have to disagree with you here. The top athletes are born with certain advantages... height, mass, higher proportion of fast-twitch muscle fibers, etc. These are advantages that no amount of hard work can overcome, because someone born with these attributes who also works hard will win every time.
What makes sports so different from intellect is this:
- Sports performance and ability are easily measured from one person to the other. People argue with the results of an IQ test all the time, but nobody argues with the scoreboard.
- Sports can be non-threatening, because if your child will not be competing, then that child's performance does not impact your child. So if someone else's kid is seven feet, and yours is five, it doesn't matter that the other kid can dunk without leaving the ground, because your kid is going to go do something else. That "something else" probably involves going to college and pursuing a good job, which puts that child in competition with just about everyone else who isn't seven feet tall.
- Being bad at sports is not considered a tragic flaw. Being bad at thinking is. As a result, the overwhelming majority of people like to think of themselves as above-average in intelligence, and they don't want a reminder when they're not.
