There's no airline outside the US with more than one major hub,
UA-DEN, ORD, SFO, LAX, IAD
CO-IAH, EWR, CLE
When a airline like BA or AF serves NRT they only have one routing LHR-NRT or CDG-NRT, under such a system larger aircraft less frequencies are better than multiple aircraft multiple frequencies.
Now when a US carrier flies to NRT they usualy offer more than one routing,
ORD-NRT, DFW-NRT, JFK-NRT,SJC-NRT
ORD-NRT, LAX-NRT, SFO-NRT
So US carriers because of their larger hub system can offer more frequencies then say European carriers for the sole reason of they have more routings, more options for travelers. In the US if a traveler in the Mid-West wants to go to Europe they don't have to fly to JFK, they have many local choices. If that same person wants to go to Japan they don't have to connect in SFO, they can fly localy from DTW, ORD, DFW etc.
If a British business traveler wants to fly to Asia there's only one option, fly out of LHR. French have to fly out of CDG etc.
The old days of Pan Am where all European flights were funneled through JFK and all Asian flights were funneled through SFO and all Latin America flights were funneled through MIA supported large fleets of 747s. Today the market is much different.
Eastern Air lines flt # 701, EWR-MCO Boeing 757