That may be it, exactly. Although you’re right that Korean is different than Japanese (it takes more room to write a word in Korean).
There’s also the fact that some people text only a word or phrase at a time (which drives me nuts, getting 10 texts in a row from someone, with a total of maybe 15 words). So if the general culture in texting in Korea and/or Japan follows this style, it makes sense that stickers wouldn’t interfere with viewing the word(s) on a small screen.
