There's some sort of r/k selection effect happening here; modern aircraft have been getting more and more sophisticated to improve survivability, which raises their cost, so there are fewer of them. Things like the stealth bombers are at the far end of that curve.

(the logical end point of that thought process is some kind of Space Battleship Yamato scenario where the designers produce an "invincible" design, but only one of it. Hopefully real planners can be dissuaded from that nonsense, although the Axis were prone to this way of thinking)