Probably because general intelligence is "the best-established, most predictive, most heritable mental trait ever discovered in psychology." (Geoffrey Miller, Spent)
"Intelligence (says Prof. Miller at page 189 of Spent) is positively correlated to:
brain size
speed of performing basic sensory motor tasks (“reaction time is a factor”, as the cop said in Blade Runner)
height
symmetry of face and body
semen quality (!)
health, physical and mental
longevity
sexual attractiveness for long term relationships"
So it is most predictive of many things, but maybe not of future real-world achievements...
Oh, people are infinitely varied, but I'd invite you to try taking away the average footballer's television, then consoling them that you'll tell them the score when it's over :)