I also expect there are health effects to lifelong withdrawal from all social contact and from not leaving one’s room. We’re not meant to totally avoid touch and eye contact and movement.
Some time it's better to be alone that to be abused. I saw a documentary about this -Hikkomori- and that was one of the guy's point. He took a lot of sht from people first at school, then at work and on the street too because of how he looked, and he got tired of having to put up with random shtty people insulting him every single day for years. It wore him down over the years and at home he only had a mother that didn't seem to be supportive at all until he gave up. You can see it in YouTube, is one of the first results. It's sad that he could not make it work by moving around until he found decent people, but some times you just have bad luck.
Meant by whom? Perhaps you aren't, but to make that claim for others is to fundamentally deny them their agency, as if they are mentally incompetent. I don't think these people are, for the most part, mentally incompetent.
Of course, such studies measure the average so each individual’s situation may be different. But it’s also not clear to me why it would be different for such large of a group.
Solitary confinement in prisons is known to be detrimental to mental health so that seems most likely to be false.
Studies might not be ideal here. How many hermits participate in social studies run at universities?
The former suggests need for social contact is in our genes. There is a reason monks had monasteries for their life of withdrawal: they still had others around.
There are rare exceptions, the eremetic monks, but overall Hikkomori do not seem content. They are perhaps more comfortable than they felt out in the world, but I strongly suspect they are in a local maximum and could live much more satisfying lives.