Would a company really be willing to help with relocation or a visa for that?
Anyway, I'm not looking right now, but maybe someday.
I don't know all the reasons. A few I imagine are
(a) they don't value programming
(b) they've got a captive workforce because Japanese is only useful in Japan so you can't go somewhere else unless you get fluent in another language and few do
(c) they still have the "one company for life" attitude. It's slowly changing but is still prevalent
(d) their education system doesn't produce people with experience. Most of the programmers I met said they had zer programming experience when they started their jobs. Instead they got the job because they had crammed for the right college in high school. They are then trained on the job
(e) they consider programming a job for youth. Many programming jobs will have an upper age limit of 35.
When I was there the two companies I worked for had an upper limit set by HR on programmer's salaries. It was 1/3 what I came home to.
Expatsian has some interesting data points.
http://www.expatistan.com/cost-of-living/comparison/san-fran...?