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.
One thing you might be clear is whether the positions are for English speakers living in Japan, or if they are open to developers worldwide.
In my experience, most companies where Japanese isn't necessary still prefer people to already be in Japan, as bringing someone from abroad is seeing as more risky, as even if the candidate ends up liking the company, they still might not like life in Japan.
If you're working for a Japanese company, you'll probably be working a lot more hours, and taking a lot less holidays than you would be in the US or UK. 4x is a low estimate for the actual amount of work you'll be doing.
You're not allowed to work in Japan without a valid visa. If they find out that you're working in Japan on a tourist visa you might not be allowed back.
It'd be a grey area (also as far as I know) to say you're freelancing/working for a Japanese company while in Japan by coincidence, and I doubt that they're very forgiving if you're sidestepping a tourist visa's restrictions.