It does not have much to do with the the dogs in Europe being higher quality. It has much more to do with the number of dog brokers in Europe. The buyers can go to the brokers and see dozens of dogs at one time. Here in the US it would take weeks to see the same number of dogs. Therefore they buy in Europe.
That is part of it. They are seeking a large number of dogs. Looking for dogs in the US with the proper drives,nerves,good hips etc etc is like looking for the Holy Grail. It is quicker to import.
They will buy from US vendors.