Slate's own Willa Paskin postulated a trend that I think is genuinely happening—better airports, worse airplane experiences:
Have airports gotten nicer in direct lockstep w flights getting shittier?-- Willa Paskin (@willapaskin) November 13, 2013
Unfortunately, it's a little hard to quantify whether airports are really "nicer," but if we accept the premise, I think we can see why it might have happened. The declining quality of air travel has to do with deregulation in the 1970s. Interstate air travel used to have its fares set by a federal agency rather than market forces, and it tended to mandate relatively high fares. That was cozy for the airlines, gave airline labor unions a lot of revenue to try to grab for their members, and generally led to a high level of service. You couldn't compete on price, but you could try to compete on quality, and unions were happy to agree to schemes featuring high levels of staffing and low plane density. Then came deregulation, competition, fare wars, price cutting, layoffs, staff givebacks, and everything else that makes plane travel annoying but fairly cheap.
You can understand an offsetting increase in airport quality as a consequence of these lower prices creating an opportunity for other players in the travel ecosystem to capture value. Every dollar people save with lower fares is a dollar that could be spent somewhere else. So now it makes sense for airport facilities to invest in quality in order to get people to spend more. If you find yourself in Chicago's O'Hare Airport, for example, it's really worth going out of the way to eat lunch at Tortas Frontera rather than running to grab whatever's most convenient. And the same factors that have driven airlines to reduce flight quality may also have driven airlines to reduce the availability of convenient scheduling options, forcing fliers into more and longer layovers in hub airports. That, too, would increase the possible pool of value capturable by airports and increase the incentive to invest in capturing it.