Not surprising really. Most US capital cities have more than one airport in a 50 mile radius. Anothet factor to consider how shoddy most US airports are in comparison to Australian equivalents.

Think about how hard it is to build a second runway, not to mention a second airport anywhere in Australis. It is a known fact that SYD and MEL are full in terms of landing slots. Curfews don't help. Accordingly where demand significantly outstrips supply, the price has to go up.

Solution: more runways/airports or other modes of transport such as high speed rail (this discussion is a wholly different cattle of fish though). Problem solved.