Am I alone in wondering why virtually the entire civil airliner sector has been surrendered to Airbus? Is there really no room for competition, either here or in the US?

For the same reason the US only has one (civil) plane builder left. The investment required to develop new planes is simply too large to be sustainable (long term) for small companies. And in that regard small is obviously relative. It is not even that long ago that the USA had 3, yes at one time even 4, builders of jetliners. Boeing, Douglas (MDD), Lockheed and Convair.