>Well, "socially unacceptable" if you're superficial and want to be popular and trendy.
The hardship of trying to break into an exclusive group ("popular") and working towards that goal ("trendy") is not too far off from that which looms over much of Korean culture. The fear that a social slight might cast you as an outlier, hurting your possibility of getting into such a group and undoing all your work, is unfortunately well founded. In the Korean language, if one is an outlier in a group, the word used to compare them to the rest is not "different" but "wrong."
You also imply that one's own sense of identity should be the main view considered, over the "superficial" views of others. While I agree on moral grounds, this is simply not the case in Korea.
It doesn't matter if Stanford verified his attendance--if it seemed fishy it was as good as true.
Korean Air stewards being required to do makeup training:
"This February, cabin crew for Korean Airlines participated in makeup training, a mandatory requirement for all new stewardesses. This year, there were more than the usual numbers in the room. Korean Air now requires male cabin crew to attend -- and to learn how to blush and bronze themselves."
"In fact, a 2016 survey by Saramin, a Korean online job portal, found that more than 60% of human resources personnel feel an applicant's appearance affects his or her candidacy."
In a country where photos are usually required with applications for jobs that are expected to last you your lifetime, would it be superficial to use makeup/photoshop in your photo? At least nobody in Korea thinks so.
Not mentioned: Korea espouses a ton of misogynistic attitudes towards women