Trust works both ways. There's also the trust that nobody will report anyone for the fraud, especially if it is widespread and normalized.

However, it would not surprise me if Japan actually did have high life expectancy rates because several other statistics seem to correlate with that, including low obesity, and universal access to healthcare.

The logical conclusion is fraud.

It was already seen over a decade ago: https://www.bbc.com/news/world-asia-pacific-11258071

I mean, clearly not all centarians in Japan are actually dead. But I think it's fairly straightforward that the numbers of super-elderly are inflated.