You are leaving out the most important factor, Europe is tiny. A flight across the entire continent is nothing compared to a U.S. domestic flight.

A flight from Seattle to St. Louis (a flight I’ve take many times and isn’t even across the entire country), is like flying from North Ireland to the Black Sea. If I took a flight to New York that’s like flying all the way to Kazakhstan from Dublin.

Distances are small in Europe which means you don’t need a lot of fuel or more importantly planes to run routes. In the time it takes American Airlines to run LA to New York one way, Ryan Air could do a round trip from the UK to Romania and back. All that leads to the only metric airlines care about, CASM Cost per Available Seat Mile. European airlines can just fly around Europe and keep a low CASM because they are all basically what would be considered a regional airline in the US.

And we haven’t even talked about distances in Africa or Asia.