IIR, Japan was a pretty xenophobic, unified, and militarized place when the Europeans showed up. And without either reputed or obvious Tier 1 riches - such as Central & South America's precious metals - to motivate a hard-fought conquest. Especially when 'most everywhere else in South & East Asia was comparatively easy pickings.
> Why do people still believe ...
Because, if you use sufficiently Eurocentric metrics, it usually was. And picking those metrics feels pretty good, if you identify uncomfortably closely with that sort of capitalism.