Originally Posted by
Eastbay1K
It is much more complicated the other way.
Everyone inbound to the USA, no matter final destination, must be admissible into the USA. Everyone flying to the USA has to go through full formalities to transit the USA. This is not the case for many other hubs. I'd imagine on a typical US-FRA flight, probably 25% or more of the plane doesn't need Schengen processing. And for those who require a Visa? There goes that connection.
Ah yeah true, I didn't think of that.