Around 1980, the 401k system began to replace company pensions (which were a complement to social security). And in the US social security is nowhere close to universal--many groups, such as police and fire, other public employees, teachers, etc., have their own separate pension systems, which is the reason social security has the infamous windfall elimination provision, WEP. (And let's leave aside how underfunded some of those pension systems are.)
In japan, there is a big divide between those who can supplement the national pension with shakai-hoken (who work for companies and can participate), and those who are not part of that. For those who aren't, which includes a lot of contract people like foreign english teachers, it should be obvious (should have been obvious?), really, from the get-go, that their pensions would be minimal/trivial.
On other forums, posts from people who don't pay into the national pension system, by choice, are not uncommon. And small businesses employing foreigners--who they assume will only be in japan for a few years--even encourage that, by saying/implying that paying into the national system is not necessary (let alone covering their employees via shakai-hoken).