Do not compare your Europe with the US. We have a lot more cars here than in Europe, and the US emission laws (especially California) are very strict. For example, in California, very old gasoline cars (before 1973) as well as diesel cars are exempt from smog check. However, all newer gasoline cars (after 1974) are rigorously subject to smog check, so any gasoline car without a catalytic converter is sure to be rejected immediately. In the next October, California will introduce Smog Check II, which requires gasoline cars to pass some extra threadmill-like dyno test while undergoing other regular smog check procedures. It's estimated that Smog Check II will cause an additional 30% gasoline cars to fail emission tests.
I had to dump (sell) all my old gasoline cars (barely passing smog check) and buy only diesel cars so that I would no longer be on the edge because of smog check schedules.
California has about 33 million people, but there are 29.6 million registered cars here!