A machine purpose built to withstand bullets does well at withstanding bullets. We could still build them to withstand bullets, but as it turns out, being almost invisible to radar is better for the survival of the aircraft.

It's kind of like how when they started issuing helmets to soldiers, there was a sharp uptick in head injuries. Suddenly more soldiers were surviving getting hit in the head.

We've made it harder to detect aircraft, and the technology for bringing them down has advanced significantly. They're much less likely to be shot at, and what they're being shot with isn't something any aircraft will survive a fight with.

There's an interesting story of a tail gunner surviving a fall from 30k feet in the tail section of a B17. The aircraft kept a steady coarse long enough for one other tech to safely escape after it had been shot to hell and everyone else inside had died. Looking very much like this image, without the tail section. But the thing is, these aircraft required fighter escorts because the were essentially sitting ducks. They were big, slow behemoths and they had to send in swarms because half of them would be shot down before reaching the target.

To become more agile, the aircraft also had to become more unstable. Meaning the technology for controlling these aircraft had to get more delicate. But not being hit at all is still better than soaking up damage. Where 30 bombers were required before, a handful of stealth bombers, or even just one, can deploy ordinance and escape the mission area without ever being detected.

This image is of a caveman crushing a skull with a rock, saying the rock is built better than a pistol that jams occasionally.

It's simpler, it's effective, but it's not necessarily better.