I know this phenonem pretty well, I searched about it and I'm nearly one but I do go out of my bedroom more often but it's getting harder. I'd firstly say that it's not only in Japan. It's worldwide and mainly due because of modernity and this society. It's known more in Japan cause as you may all know, Japan is more advanced in terms of modernity and technology. I don't pretend to be right, it's just my opinion.
I do think people getting into it are mainly the sensitive ones (a.k.a the "weak" for this society). I mean, seriously, I fully understand them : who wants to work to survive without any purpose nor liberty nor enjoyement. Modern slavery at its peak..
It's kind a way of protestation for people who don't have enough strength and purposes to say anything. The people who are tired about everything. (Please keep in mind that's it's my opinion as a nearly hikikomori)