Other than safety, why haven't jet aircraft improved since the 1950s?

Here are the specifications for the de Havilland Comet 4 from 1958:

Length: 33.99 m
Cruising speed: 840 kmh (520 mph)
Passengers: 56-81
Range: 3,225 mi (5,190 km)

And here are the specifications for the new Airbus A319neo introduced in 2016:

Length: 33.84 m
Cruising speed: 833 kmh (518 mph)
Passengers: 140-160
Range: 4,318 mi (6,950 km)

Literally the only difference is that 1.) the new plane can travel about 1/3 longer on a full tank of fuel and 2.) airlines have managed to cram about twice as many passengers into the same amount of space because they know that people will always buy the cheapest tickets no matter what and any airline that tries to actually make flying a better experience will quickly go out of business in an industry that's a race to the bottom with razor-thin margins.