"Outside Japan, there are just two case reports of hikikomori syndrome, one in Oman and one in Spain." I agree that this should be quite common around the world. In the U.S., there are a lot of 20-year-old men who live with their parents or have a part-time undemanding job and spend most of their time playing video games, etc. and don't intend to get married.
Most/all of what you describe is not hikikomori. The "pure" form of hikis will rarely even leave their rooms. The idea of a part time job is very much not something a pure hiki would do. There is a lot of social maladjustment in Japan -- I've never seen this to the same extent in the US or EU. It might be possible in some middle eastern countries due to extreme religious reasons (e.g., Oman), but the underlying reaso…
I do know people with panic disorder (and the associated agoraphobia) and social phobia who live on a "Non24" calendar (they might call you at 2am in the morning but not be awake at 3pm.) Depression, anxiety, drug use all compliate the equation, but it's hard to believe that this phenomenon is all that different from hikkiomori.
Many cultures take pride in an imagined exceptionalism and Japan is one of the leaders in that. It serves their ideology to believe that they have a disease that others don't.
I also have the impression that many people have subclinical agoraphobic or social phobic disorders. For instance, my dad was high functioning in many ways but he absolutely refused to go to any kind of retail space, including the store my mom worked at.
Social phobia has a strong association with bullying; some people survive bullying pretty well, but it has a devastating effect on many. Behind the misogyny of the tech culture you might find deep anger about how many smart people were shut out socially in high school. Some people find a pillar of stability that helps them get through it, but the experience of being in a hostile crowd can damage you profoundly.