Por que os EUA têm tantos aviões de aviação geral?

Combination of two factors

1. Wealth, particularly long term. The USA is a wealthy country that has been that way since the invention of flight. A large number of general aviation aircraft flying today are quite old 70's and 80's equipment makes up a large portion of the market and even 40's and 50's equipment can be found easily.

2. Area and population density. Europe and the Asian Pacific are densely populated areas. Cities are close to one another meaning commute times are short and airspace is highly regulated (those airports are close together as well). This makes it harder to fly and leaves less incentive as one is less likely to have a particular route in mind.

The only a few wealthy countries have large amounts of land, Australia, Canada, and the USA.

It is said that airplanes fly because of money and the Bernoulli effect, in that order. So the first cause is simply that there's a lot of wealth in the USA to support what is essentially a rich person's hobby. There are also a lot of airplanes still flying that were built during the boom years of aviation to support interest in the novelty of plane ownership (which is much less a novelty today), which tends to hold down the purchase price of a small plane. In addition, gasoline is much less expensive here than in western Europe so the operating costs will be lower.

Part of the reason is also historical. After World War II was over there was a large number of veterans with flying experience returning to a country that suddenly had a massive surplus of small trainer aircraft.

While the US expended huge amounts of resources on World War II, it was largely untouched by the destruction that many other countries suffered. This meant that instead of having to rebuild, the US could focus on growing the economy. Though GA declined in the Great Depression and was suspended during World War II, there had been a similar situation after World War I that had gotten the industry started.

Small aircraft were much simpler then, with a much lower cost to build and certify. This made general aviation relatively much cheaper. Also, while the US certainly had a highway system, the interstate highway system had yet to be built. Commercial aviation service was not nearly at the scale it is today and was still fairly expensive. This made general aviation a much more reasonable option for traveling around a large country.