You are correct, Europe is a small fraction of the U.S. market that in calendar year 2000 had 668.5Million passenger enplanements and 27.05Million aircraft operations from the 100 busiest airports alone.
This shows the lack of economic incentive for the U.S. to negotiate an open skies agreement with EU. Some kind on an open skies-ish deal with individual countries or EU as a whole maybe, but why give foreign carriers entre into a huge market in exchange for entre into a dinky one?