Add to that the fact that a lot of airports either came into being more or less from scratch, or underwent massive expansion in the 10-20 years right after World War II. All of those facilities (or at least the ones that remain in operation) are getting old, and getting crowded (see above) at about the same time...which means that you should see a 'bubble' of terminal building / modernization / expansion at some point.
All that said, while I don't think the spate of new construction is any real sign of a conspiracy, and I'm not very sold on TOMIFC (The Obligatory Massive International Freemasons' Conspiracy), there is something down-right bizarre about the new Denver Airport. It's got to be the product of *some* conspiracy...at the very least, a cabal of 'artists' who conspired to bilk the public for a ton of cash, or a conspiracy of gastrointestinal specialists who are gonna make a ton treating the apocalyptice cases of


