Also, there are many types of hikikomori. Some have no family support or live with them.
I know one who lives in an internet cafe, alone, and has for years. The staff told me she avoids everyone as if they were toxic poison. Only comes out at night for bathroom trips / snacks and she will run back into the cubicle if anyone's in the hall when she comes out. It's been three years now. In a cubicle. Leaves the payments near the drink section in an envelope always at the same time of night. Doesn't even use the cubicle computer, she works out of her smartphone. ( Our company actually hired her for a while. ) Most employees do not even know what she looks like, I certainly don't. She's adamant that she wants to live like this.
The son of a friend has been in his room for the last ten years. Does nothing all day. Tried to kill himself several times. Their life is an absolute nightmare.
Etc. There's no one-size-fits-all, and in some cases I don't know if mental illness has anything to do with it.
Edit : Our next door neighbour, as far as I know, has not been out of his one room apartment in at least 8 years. He's old ( 80s ? ) and a couple of volunteers check on him once a week and bring him food, otherwise he'd be dead already. We tried to talk to him once and were told not to do so as he could turn violent, "he's a hikikomori"... It took a while to find volunteers to help him out that he would not attack, apparently. I mean, the list is awfully long. I could go on and on.