Im curios if population desnsity may have any corelation to this or not. Im also thinking that the culture is shunning a certain personality type or people with some mental struggles that their only way to adapt is to withdraw.
Key sentences (1:30): "Young ones found themselves born in a world with far more mice than meaningful social roles. Males faced a lot of competitors to defend their territory against. Many found that so stressful they gave up. Normal discourse within the community broke down and with it the ability of mice to form social bonds."
Very nicely illustrated.
Humans are social animals, but we aren't ants. We're not supposed to live in concrete jungles. As others have said, hikikomoris are one extreme of a continuum of negative outcomes caused by modern life. A low birthrate is another, which is correlated with urban living (there are other factors but this trend is unmistakable).
I believe this is a natural outcome of very high population density, a negative feedback mechanism. When life becomes unbearable because of too many people, we withdraw and don't reproduce.