Why is it so hard to build an airport?

Your definition is accurate, but I think it is incomplete. The stereotypical NIMBY doesn't want something in their backyard, but they still want the advantages of it existing. They want it to be place in someone else's backyard.

People opposing airport construction still want to fly. They just want someone else to have the noise.

Right. That factoid is only true because cities in the US tend to avoid mergers.

The US had 62 statistical areas with a pop. over a million in 2020 (plus San Juan, PR).

> US airports always seem to be surrounded by densely populated residential areas

The US in general is very densely built up. The distinct lack of green space in US cities is very noticeable to the non-US observer.

So I think its more of a case that US airports are surrounded by "less" densely populated areas.

> And secondly Us airports always seem to take huge plots of land compared to Euro airports.

With the exception of the limited number of buildings of historical interest, In general the US doesn't do history in its buildings. They'll happily tear stuff down and build a shiny new thing in its place.

As I understand it the whole concept of green belt / urban conservation is also fairly minimal in the US mentality, so large land grabs for building and expansion of airports are easier in the US.

Meanwhile, in Europe, most airports have history and grew organically. And the whole green belt / urban conservation thing is much stronger in Europe. For example, London Heathrow started off life as a single grass runway with a few simple buildings and grew organically over time. Its growth ultimately limited by the city that grew around it, and so you end up with for example the present controversial discussion over the possibility of a third runway. I would hazard a guess that if Heathrow were in the US, the third runway would have been built and operational by now !

Counting "city population" is nearly a pointless exercise. What matters is the metro area, otherwise you get some pretty useless results. Austin, TX is tenth by city size in the US, with just under a million people, while San Francisco is 17th, with about 800k. But Austin is a much bigger percentage of the total Austin metro area, while the Bay area is ~8-10 million people total. As such, SF feels like a much bigger city in nearly every capacity.

> And an airport can’t be too far from a city and remain useful, since travelers need to access the city, workers need to be within commuting distance, and so on. In Canada, Mirabel airport was built 35 miles from Montreal, surrounded by a 79,000 acre buffer zone to prevent any issues of incompatible land use. Mirabel was expected to replace Dorval (today Montreal-Trudeau) as Canada’s main eastern airport, but, in part because of its long distance from the city, this never happened, and Mirabel stopped serving passenger traffic in 2004.

Another relatively new airport built far from the city it serves is Munich airport, located around 33 km (20 miles) from the city center, opened in 1992. The two major candidates for "relatively sparsely populated area" when the airport was planned (back in the 1960s) were a swampy area north of Munich (Erdinger Moos) and a forest to the south (Hofoldinger Forst). They picked the swamp, which leads to frequent fog problems. And they solved the problem of the old airport competing against the new one by simply closing the old airport (the company operating the new one is the same as for the old one, so no protests there). Some equipment was even moved from the old airport to the new one in an overnight relocation. But, as the 30 years from planning to opening show, even this remote location was not without conflicts. And, more than 30 years after the opening, there is still no fast train to the airport. The Munich-Nuremberg high speed railway line could have been routed by the airport, but (according to rumors) this wasn't done to protect the Nuremberg airport. Then there were plans for a maglev train (Transrapid) which were cancelled in the early 2000s. Currently the plan is for an Express S-Bahn line, but since the S-Bahn tunnel in the city center can't accommodate any more trains, this will only be possible when the second S-Bahn tunnel is completed (the date for that keeps getting pushed back, currently it's 2035).