system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it cheaper to fly to different countries in Europe as opposed to it being expensive to fly to other states in the US?

Traveling around Europe is easy and moderately priced. Flying across the US can run you 400$ from NYC to Indianapolis (for example)
gpt
Europe has a well developed train system which competes with the airlines.  

This drives down prices, and eliminates routes that are not profitable enough.  The flights that do exist will be cheaper, but there will be fewer of them.

Also, some European countries have national airlines that are subsidized.