Unless you live on an island, traveling doesn't have to be by plane. Here in Europe many youths get an Interrail pass[1], and you can get to Africa with a short ferry crossing.

[1] https://en.wikipedia.org/wiki/Interrail

I think you have it somewhat backwards though. Europe is small. Unless you live in Europe, you are quite restricted in terms of how much variety you can experience without flying. For example, a trip from San Francisco to Los Angeles takes over 6 hours of driving (bus is even worse). And that's "just next door" as far as US metropolitan cities go. And no, this isn't americanism or defending car culture. Consider also that rail system coverage in South America or Africa is severely lacking if compared to Europe. And for better or for worse, a very large number of popular destinations are indeed insular.

Ironically, Europe is also effectively inaccessible without planes for tourists from elsewhere (either because of geography or sheer distance)

You don't need to fly around the world to see many of the benefits of traveling. Perhaps more Americans should drive down through Latin America - and I don't mean partying in Cabo. Perhaps more Americans should drive around the country.