Probably cultural - if a society celebrates physical prowess (sports etc. especially for males) and physical appearance (fashion etc. especially for females) but doesn't celebrate intellectual prowess for either gender - often the opposite - then children, teenagers, and young adults won't view working hard to improve their mental abilities as something worthwhile that they will be rewarded for. It's a symptom of a fairly unhealthy society.
There are a great many historical reasons why this situation arose - aristocrats feared the rise of well-educated groups within the serf class that might challenge their power, and plantation owners famously forbid teaching their slaves how to read and write, and some of that thinking persists to this day.
Regardless, people who spend roughly equal time on developing both their mental and physical capabilities via deliberate (and time and energy consuming) practice are the ones that tend to turn out healthiest and happiest.
There are a great many historical reasons why this situation arose - aristocrats feared the rise of well-educated groups within the serf class that might challenge their power, and plantation owners famously forbid teaching their slaves how to read and write, and some of that thinking persists to this day.
Regardless, people who spend roughly equal time on developing both their mental and physical capabilities via deliberate (and time and energy consuming) practice are the ones that tend to turn out healthiest and happiest.