Why not more direct routings?
well, the airspace still has a lot of restrictions. One of them is military airspace. If the guys have to play war, you got to fly around them. The current airway system is planned around these military areas.
Then traffic doesn't just go from A to B. It also goes from C to D and from E to F. These paths will cross each other in some way, putting a great workload on ATC. Keeping them on a common routing will ease separation.
RNAV and GPS are commonly used in terminal areas, on SIDs and STARS. These RNAV flight paths have greatly relieved congestion and shortened flight time due to their great variety, offering entries /exits from almost everywhere.
Also, a lot of todays airways are established on so-called "waypoints" which are in increasing numbers plain RNAV fixes without any navaid relation. So RNAV has already increased the number of available routings.
Also, ATC needs some planning basis, so the flight plan of your flight will be filed along airways. You will always be cleared along these airways during the clearance you recieve in the pre-flight phase. Then, according to the traffic situation and workload, ATC will give direct clearances.
The situation will slowly change with the introduction of FANS. This will enable more so-called "free-flight", proceeding on direct, great circle routings. This system is still not totally defined, then it requires completely new infrastructure, starting with new ATC systems, new air-ground data transmission, new technology in the aircraft themselves. This will still take years and even then will be introduced only regionally (North Atlantic, for example), as it will cost vast sums of money.
So, the current airway structure will be around for quite some time to come, supplemented by direct routings depending on the overall situation.
Just a couple of thoughts on the topic......