I think the Hikikomori phenomenon has been widespreading due to its popularization. imho being a shut-in has become a codified cry for help, a way to express a certain discomfort backed up by social proof. It has nothing to do with being lazy or spoiled, as implied in Japanese media like the Welcome to the NHK anime; I've yet to read the novel.

Hikikomori exists outside of Japan too. I recommend you to watch the following video about hermitism and shut-ins: https://www.youtube.com/watch?v=70bv5gaN4LI