See, your realization would actually be a good shower thought.
OP's post seems more like a response or reaction to something else (which we don't have context for). Who thinks that North/South Korea, China, and Japan are the only countries in Asia? Like, specifically in the US, I think we're all familiar with Vietnam (and the Vietnam war) along with Laos and Cambodia, the Philippines, Thailand, etc.
I don't mean to demean anyone not familiar with geography, but even if you don't know the names of the countries, you'd have to know more countries exist there, which is why I think this is a weird post. It'd be like posting "The United States consists of multiple states"