The shut-ins suffer from an extreme form of social anxiety and the symptoms usually begin in adolescence or even in childhood. In addition, the Japanese do not encourage gregariousness (though they do encourage politeness and consideration) and do value their shy children
The "parasite singles" are a different matter. In Japan, there is no shame in living with your parents until the day you get married. In fact, it's expected. So yeah, there is a "parental" aspect to it, but it's something I see as more of a cultural matter than a parenting issue.