Why do Koreans say 웃어른 instead of 윗어른?

ANSWER: Because there is no 아랫어른.

The pure-Korean word 위 means "up" or "top," and the pure-Korean word 아래 means "down" or "bottom," so the word 위아래 can mean either "up and down" or "top and bottom." The words 위 and 아래 need each other to have any real meaning because they need the contrast that each gives to each other. In other words, without an "up" (위), there is no "down" (아래), and without a "top" (위), there is no "bottom." So, when you see a word that uses 위, you can expect that there is a contrasting word that uses 아래.

When Koreans want to create a word that uses the meaning of 윗 without also creating a contrasting word that uses the meaning of 아래, then they use the suffix 웃 instead of 윗 since 웃 does not imply there is a contrasting word that uses the meaning of 아래. In fact, 웃 implies there is no contrasting word that includes the meaning of 아래. So, the word 웃어른 implies there is no such word as 아랫어른. And 웃국 implies there is no such word as 아랫국.

The word 웃어른 literally means "upper (웃) adults (어른)," which translates as "one's elders," and "elders" are adults who are much older than oneself. So, since there is no word that means "lower (아랫) adults (어른)," Koreans use the word 웃어른 instead of 윗어른.

In the above definition, noticed that below the definition for 웃어른, there is a definition for 웃옷. The word 웃옷 is used to refer only to "an outer garment" or "a coat," not to "an upper garment," so the second part of the 웃옷 definition is wrong. For "upper garment," you must say 윗옷 because there is the contrasting word 아래옷, which means "lower garment."

Actually, it seems that Koreans are still trying to figure out when to use 윗 and when to use 웃. For example, my 1998 Donga's Prime Korean-English Dictionary lists both 웃물 and 윗물, but my 2014 Minjung's Essence Korean-English Dictionary lists only 윗물, probably because they have learned that 웃물 would imply there is no 아랫물, but since there is an 아랫물, 윗물 should be used instead of 웃물.

So, just remember this: When there is a 윗, there is also an 아래 or 아랫, but when there is an 웃, there is no 아래 or 아랫, at least there shouldn't be. But some Koreans and even some Korean dictionaries still need time to catch up.