Because the truth is that American cars are much bigger, heavier, and generally safer than European cars. Numerous, (rather popular!) European cars that get excellent fuel economy (better than 40 MPG) simply cannot be imported because they'd miserably fail the crash tests.
To be sold in America, new cars must have a crash cage around the passenger compartment, around which are various crumple zones that absorb impact and improve passenger safety. Crash cages, by design, must be very strong in order to prevent passengers from getting crushed by stupid amounts of energy. This makes them heavy, and that makes it darn hard to get decent fuel economy, especially in stop & go traffic. (weight isn't nearly as much of a penalty on freeways, particularly on flat ground, though hills steep enough to require braking on the downhill runs can get rather inefficient rather quickly)
Bullshit. All cars sold in Europe follow safety standards equivalent to those in North America, and have had 'crash cages' for nearly 50 years.
The main difference is cultural expectations about car size. As an example, the Ford Focus, what Americans consider to be a 'small' car, has 2 models smaller than it in Europe (the Ka and the Fiesta), and the Focus is considered here in the UK to be a 'medium' sized family car. The Ford Mondeo, very similar to the Fusion in the USA, is considered to be 'large'. Ford do not sell a larger car than the Mondeo in Europe.
Engine size expectations are similar. Here a 2.0L I4 is considered a normal, reasonably powerful engine. An 'economy' car would have a 1.4 or a 1.6. A 3.0L V6 would be considered a 'fast car' here. V8s are almost unheard of.
Amusingly European cars generally have faster top speeds than American cars, despite the smaller engines. Some years ago I took an American on a road trip (along with other people) and he was amazed that my 1.8L Ford Escort could achieve 135mph. Here if a V6 can't get to at least 150mph it's considered to be a bit crap.