Sometimes US-Europe roundtrip flights are cheaper than domestic flights. Without the cost of baggage, selecting seat, food, using the bathroom, etc. Pretty ridiculous.
Americans weigh 15-20lbs more than the average Euro More weight equals more fuel. Tell people to lose weight and they get offended...so airlines keep quiet and charge more. People still b****, but you can't tell people the truth cause they get all offended.
Sorry, but that's not the sole reason. I lived in the UK for a couple years and flew everywhere for under $150, including distances much further than what I fly in the US. Flew to Marrakech for $90. Cant even fly to Dallas for that.