I'm certainly not an expert, but I suspect it might have something to do with Korean society. I don't remember the exact name for them, but I watched a video on how every year there are these exams Korean students take, and literally, flights will be grounded, traffic will be diverted and teachers will wear certain shoes to muffle their sounds. It's literally a group effort on a mass scale just to ensure the students do well and don't get distracted. Plus, Korea has a pretty blatant social hierarchy with terms given to this group of people and that group and so on, as well as rules on how to address people who are older and younger than you and so forth.
K-pop idols don't solely make up the landscape of Korean music, but it's no wonder that there's such a huge market for them when group efforts and social structures are the norm in Korean society. It would make sense then that people would enjoy seeing groups succeed and work hard together, dote on their maknaes even if they're only, like, a year or two younger than their second-youngest member and rely on their leader to be the grown-up who takes care of all the grown-up things, like shopping expenses and whatnot.
It's also clear that the k-pop industry has a lot of government support since they're what gives South Korea the bulk of its soft power. So maybe the government wants groups to be the face of the Korean music industry in order to show the world exactly how the country's culture can mesh beautifully with music and entertainment.
Again, I'm no expert. And all of this is purely conjecture. But even in the US, where I'm from, isn't it sort of similar for music to be a reflection of society and for people to like that? I'm thinking of shows like American Idol and The Voice and just the fact that we have so many solo artists who come up through Disney or through other means. Here in the US it's all about 'picking yourself up by your own bootstraps', which can get pretty toxic, ngl (tbf, any societal structure can be toxic when taken to extremes), but people like seeing solo artists succeed because of this mindset, even if they're not consciously aware of it.
So I don't think it's too far outside the realm of possibility to say that k-pop groups work because they're also a reflection of their society.
But again, this is all just a theory.
I really need to stop writing essays, wth am I doing.