Because in those days, automation cost more than labor and was less reliable. In todays world, (first world) labor costs more than automation and is in many ways less reliable.
[Diversion which I’ll connect at the end]
In aviation today the big push is on pilotless aircraft. Not simple drones or overgrown remote-controlled models, but largely autonomous fighters & bombers. Google [UCAV], [UCAS], or [UCLASS] for more. These are research projects, but the expectation is that they will produce real functional front-line equipment in 20-30 years.
As **Francis Vaugn **said the challenge is not normal operations; computers can do a decent job of that today. The challenge is entirely building in enough of the right kind of redundancy & degraded modes for the computer to see and understand the true state of the machine, malfunctions and all.
In aviation today, we have layers of equipment & instruments ranging from real fancy to real basic. The critical thing the humans provide is the ability to effectively diagnose failures in real time and cut over to at least partial reliance on backup systems. In the face of ambiguity, the humans don’t always get it right immediately, but they can adjust their plan as the situation develops.
The jet I fly has (no exageration) 120 idiot lights, 40 aural warnings, and umpteen indicators. When half that stuff lights up at once, sorting out what’s important and what’s not is a challenging problem for AI. A simple fault diagnosis tree is too rigid.
Eventually the state of the art in AI will get to where this is doable enough for sorta-expendable military aircraft. I doubt I’ll live to see it for machines where the customers riding on board want near-total assurance of arriving alive. But eventually airline pilots will go the way of elevator operators and subway drivers.
A big difference between trains and airplanes is the impact of weather. If we could wave our magic wand and make weather disappear, about 90% of the headaches & instability in the current aviation / airline system would disappear. Except in heavy snow country, trains don’t much care about weather.
Remote control locomotives are commonplace now. Mostly-autonomous ones would be comparatively trivial versus the mostly-automonous airplanes being built now. Societal concerns about having that much concentrated power rolling through towns controlled only by “blind” machines will hold back development far more than technical hurdles will.