Population density is absolutely the driver of all of this. It increases competition, decreases personal space, and limits access to nature. Humans are social

Population density is absolutely the driver of all of this. It increases competition, decreases personal space, and limits access to nature. 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.