This is largely my anecdotal experience, but having lived for an extended period in both places (and have lived with Europeans in America for extended periods), I have noticed that Europeans generally wear black socks to work out, or hang around, complement sneakers, etc. Is this a cultural thing? Why do American men generally wear white socks for casual/athletic dress, while Europeans generally wear black?
What led to the rise of the white athletic sock in America? Isn't it harder to wash? Or, conversely, why do Europeans wear black socks, a color generally reserved for more formal stuff in America -- and that tends to clash with white/light shoes -- for exercising?
White socks were recommended for decades as a way to help cure or avoid athlete's foot. Those may have been the only readily available all-cotton socks (another recommendation) in that era, or it may have been that white socks could be laundered at higher temperatures, using bleach, to prevent reinfection.
Fashion also became reinforced by observation, and for Americans who came of age in the 1970s and 1980s, white socks are worn with athletic shoes—and are worn at no other time. Only nerds wear white socks with street shoes, or wear dark socks with sneakers.