Why are 20 somethings without jobs, with minimal face-to-face social skills, average to poor grades and therefore minimal career prospects, confined to their homes when a hyper-competitive, youth loving, cute/adorable/flashy/shiny-thing loving society awaits them outside?
When the alternative is to be involved in the antithesis of your own persona, I can imagine why the slightly less horrifying self-imposed withdrawal is still more attractive than the slightly more horrifying "outside". Despite the blatantly obvious effect it may have on your life.
Source: 2 Close pals in Shiogama living together (one who had to deal with the other's hikikomori ways).