The US is much more spread out than Europe, much more. A flight between the major cities in Europe, say London to Paris will be maybe $100 one way. A similar trip (distance wise) in the US from New York to Boston would also be $100 one way. A lot of people in the US fly much further than that. A trip from New York to Phoenix Arizona (not even all the way across the country) is a similar distance to Lisbon to Helsinki and will probably cost about $400 for a round trip flight in the US, there and back again.