To understand this, you have to remember that the national airspace system and modern jetliners did not just appear one day in their present form. It has been an evolutionary process.
In the old air mail days, pilots navigated visually - following rivers, railroad tracks and flying over known cities and towns. This worked fine during the daytime, but was useless at night. When the demand forced air mail flights to fly at night a series of bonfires was lit along the route of flight. There needed to be someone there to light the fire, so the flight paths tended to “wander” along the ground to coincide with places that had at least some people on the ground.
Even bonfires and railroad tracks couldn’t help pilots fly in clouds, though, so something else was needed. Eventually radio navigation evolved. A radio transmitter was placed on the ground and the pilot tuned it up. Initially this was a very crude system, as you had to listen to the volume level increase or decrease to tell if you were going toward or away from a station.
These radio transmitters were also placed near cities and minor population centers. Not only did it help make maintenance easier, but often the town was the destination for the aircraft, and you now had a way for the pilot to navigate directly to your airport.
As the radio beacons got more sophisticated so did the aircraft. Now the beacons could drive a pointer in the cockpit, making navigation much easier. Easier navigation meant more reliable transportation, and air traffic increased. After a series of mid-air collisions the government realized that a system was needed to separate aircraft not just around airports but also all across the country.
They came up with the national airspace system. Airways, often called “highways in the sky” were plotted out. These airways provide a way to fly between two radio beacons. For example, fly outbound from the Los Angeles beacon on the 080 radial (an imaginary line that extends outward at a 080 heading from the beacon) until you intercept the 260 radial inbound from the Needles beacon. This allows you to know your position the entire way, and you are able to stay on course. It also allows for airways to be constructed from beacons that are very far away - you don’t have to pick up the Needles beacon right after takeoff, but only after flying outbound from the Los Angeles beacon for quite a while.
By having aircraft stay along defined paths in the sky, many more aircraft could fly safely in the same airspace. The route system became a valuable tool for Air Traffic Control - they could route aircraft in ways to make the system more efficient. For example, a “corridor” is developed for all aircraft coming in to New York from the south. A different one exists coming from the west, and still more exist for aircraft departing New York.
As traffic increased the beacons got better, becoming VORs (Very high frequency Omni-directional Radio) that included DME (Distance Measuring Equipment). More and more were activated around the country, but the practice of routing aircraft over these beacons continued.
Today we have the technology to go direct to anywhere (inertial reference systems and GPS are the two most prevalant technologies). However, the same limitations concerning traffic flow remain - you can’t have 300 airplanes converge on JFK at once. The FAA is working on what it calls “free flight” which will allow pilots to spend most of the flight going direct to their destination, only jumping into an arrival corridor near the end. This requires them to completely re-work how they accomplish en-route separation, so “free flight” has been the perpetual “next year’s” technology.
So it comes down to the national airspace system being an evolutionary technology rather than a revolutionary one.