You're missing a key point here.

Of course there's more people travelling to neighbouring countries than non-neighbouring ones. Simple cost explains that.

But there's no real meaningful increase in costs between flying to Europe and flying to Asia, say. Doing a quick eyeball check, ticket prices from NYC to Seoul, for example, are about $400-500. Flights from NYC to Frankfurt are about $200-300. South Korea, however, is significantly cheaper than Germany for tourism. Depending on whose numbers you go by, SK is anywhere from 33% to 50% cheaper than Germany for tourism, so the extra $200 for the tickets is more than made up for in literally two days of actually being in the country.

Costs scale roughly the same across Europe, and SK is kind of the middle of the pack costs-wise for Asia, so it's a decent proxy. (Japan is more expensive, China and Vietnam are cheaper.)

Yet far more people fly to Europe, when they bother going beyond North America, than to Asia. Meaning far more people are flying to places that are culturally not that different from their own, comparatively, despite the fact that the latter is cheaper. By far.

(How far? A friend of mine in Canada came to visit me for two months one summer and worked out that they'd turned a profit, despite the then very high air fares, after the first month just in savings on electricity and food costs living at home.)

USAnals are very parochial (40% never having had a passport), and when they do step outside of their parochial bounds, they go to places that are very similar to home anyway, even when places that are actually different cost less.