Years back, I had a car that went from a bazillion miles on a distributor cap and rotor to dying with just a few thousand miles on them. Years latter, the same happened with my cargo van. The solution was the same for both rigs.
Trouble shooting it, I did a lot of research. The thing that showed up, over and over again, was, the distributor was old and worn. More specifically, those little gears at the bottom went from square ends to knives. They, literally, were sharp enough to cut something.
For those unaware, the reason that was a problem is, that threw the point at which the system fired a plug off. A bit after when it should have.
Okay, there is that, but the details as to why this was shortening the life of the cap and rotor were, in all sites discussing it, lacking. Mainly because all but a few did not know the whys of the problem.
I pondered it a bit, then remembered I used to make my living playing with electrons for the federal government. In the course of getting there, I had to learn a little about electronics (components, etc.). One of those things was, coils hate change. They will fight it. To that end, they keep raising voltage to keep the circuit going. That is how coils and points, or point replacements, work. The voltage keeps climbing until it jumps the plug gap [and the system is designed so it will do it at the right time].
With that aforementioned distributor issue, the worn gears of the distributor caused the system to fire, say, 1/4" past the terminal, instead of where the system design would have had it fire, wrecking havoc inside the cap (e.g., burning, corrosion), where sparking should be kept as minimal as possible. The major sparking was the opposite of great for the life of the cap and rotor.
Then there is the matter of that additional gap inside the cap on top of the gap at the plugs meant the coil, doing what coils do, rose the voltage even higher than normal, until the spark could jump both gaps, or find another, shorter route to ground.
And the problem grows from there:
I had put new, high end wires on the rig, then ran it in the dark of the night, so see if I could detect arcing anywhere. I did. The whole set of wires went into corona mode. I was pissed, thinking NAPA was pawning cheap dielectric strength wires on the public. That was when I remembered that coil, volt climbing thing. What was happening was, a system designed for, say, 50k volts was suffering 70kv in operation. In other words, too much for even quality wires to insulate.
In the case of that cargo van, there was even corona around the coil.
With the new distributor dropped in, the corona went away, and caps and rotors went back to lasting 50k miles.
ON A SIDE NOTE OR TWO:
(1) Because there was corona around the coil of the van (different kind of coil than our square bodies), I just assumed it took a beating, so swapped it out too, rather than risk a short and being stranded on down the line.
(2) Those higher volts (in the thousands of volts) aren't nice to anything in the ignition circuit. There could even be internal arcing in the coil itself, which would be especially problematic for electronic coil [e.g., HEI] systems.
Years back, I looked at an electronics circuit that had been hit by a static discharge (one just made by rolling an office chair across a floor) under a microscope. Saying it fried the tiny circuit was an understatement. It looked like the Shaw of Iran HAD lived there.
