I might be way off, but I tend to view this from an economic perspective in that these young (and not so young) shut-ins, in a pragmatic sense, don't need to work to survive. Their parents are able to provide for them all the basic needs for life - mostly shelter and food. It is a relatively small marginal expense to the parents who usually already have fair incomes. Until the parents pass away, which may take some time with Japan's famously long lifespans, most of these shut-ins will probably be alright.

Another aspect I find interesting is that the article is written mostly from an extrovert's perspective on culture. For an introvert, it seems like paradise to be a shut-in. I get fed, cared for, and I can focus on my internal life and interests, with no need to deal with others and all the problems they bring. As an introvert focused on myself and my own well-being, there seems nothing wrong with being a shut-in. Of course there are macro-economic arguments against it, but from a micro-economic point of view where I only care about my own needs, I would be satisfied with a shut-in life.