Air travel is easier to make work because most of the infrastructure is at the airports, not on the routes. Airlines can change routes easier than railroads. The speed of air travel has natural utility. Commercial airplanes come in many sizes... Turboprops can reasonably service small airports that can't fill a 737. Once daily or even a few times a week service from a middle of nowhere airport to a hub opens you up to a lot of destinations with 1 stop, and a 2nd stop (maybe with a different airline) probaly gets you to anywhere you want to go.

With small airports, there's probably plenty of flight time is worse than drive time and security and rental counter time add up too, so flying isn't always less time than any other mode, but often it is.