within the last 20 years. Also, they were built with an eye to the future so the original plans already included plans for how the airport would expand, so upgrades, expansion and continual maintenance to keep everything up to date are much easier to do. US airports are all significantly older and demand/utilization has greatly outgrown the capacity of any original plans, even if those original plans included expansions like the Asian airports.

The cost of building a new airport to replace LAX, or any other major US airport, is probably way higher than would make sense, and there is not a real good place to put it because of how much southern Cal has grown. For most airports in the US, we are kinda' stuck with what we got and cities have to figure out how to upgrade and improve within their existing footprints, all while maintaining current capacity. Chicago has been doing that with O'Hare's Termina 5. It sucks during construction, but no other options and it (should) be better afterwards.