Probably because general intelligence is "the best-established, most predictive, most heritable mental trait ever discovered in psychology." (Geoffrey Miller, Spent)
http://www.barrelstrength.com/2013/12/31/ramachandran-iq/
where there is:
"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"