Different market dynamics. And just looking at cost/distance doesn’t work well. Many short haul flights in the US are crazy expensive because of supply and demand (regional service on UA out of DEN or ORD is a good example).

Also: in Europe, the LCCs have to compete w railways — not so in the US, where airlines largely don’t compete w Amtrak (except in the Acela corridor).

Competition, supply/demand, AND distance (which is a cost function) all impact pricing. And of these three factors, distance is often the least significant variable in the model. Example: the ASM costs in the NYC-LA market are higher than, say, Denver to Billings, MT. But the amount of competition and demand/supply in these two markets varies dramatically, and therefore there are often cheaper fares to fly 2,475 nm from NY to LA than 455 nm from Denver to Billings.