I think there are complex factors, which almost certainly vary according to each case. My youngest hardly leaves the house, but he has a number of mental illnesses, and the withdrawal from society started when he was a young teenager (unable to attend school and so on due to serious anxiety disorders).
Yet, I feel, as parent(s) (possibly usually the mother) we can enable this behaviour. The enabling is nothing to do with spoiling the young person, treating him/her like a golden child and so on. It is often because the young person is accessing no other help at all, and if they were not gaming, they would be hiding from the world, under the bedcovers, engaging in multiple rituals and so on. It is enabling because turning off the internet, not providing any resources and so on, does not work. The internet is often their primary coping mechanism, and unless we are prepared to watch the young person starve, we will provide food for them.
Furthermore, as a single parent (and whether we have partners or not, often we function as single parents in these cases), spending hours, and I mean hours, trying to get a young person to leave the house, even for the fifteen minutes it takes to sign on for benefits, is emotionally and mentally draining. Add to the mix the fact that the parent may work from home, and the situation becomes even more locked in.
The only positive so far, from my son's hermitage, is that he has access to the internet, and he has started to communicate with people who have a similar mental illnesses. He shares his communication with me, and it does seem to be helpful. I hope that through the positive responses he receives, and perhaps finding some online validation, he may start to recover. May be he will venture out one step at a time.
So, yes, there may be some people who cut themselves off and remain at home as a lifestyle choice. However, for many, I would guess there are complex mental health issues and possibly (undiagnosed) neurodiversity at play.
I apologise for the essay.