[MindSpunk]

They're designed around not getting hit at all, rather than being able to take hits. Stealth, stand-off weapons, sensor fusion and information displays all so the plane never gets put in a position to be hit.

That's not to say they don't defend in depth, one reason twin engine fighters are desired is because of engine redundancy after all, but a more "armored" plain is a slower, bulkier, easier to detect and easier to hit target. And you'll still likely get taken down in one hit.

And there's still not a lot you can do if your engine swallows a bird or two, especially if you only have one.

The military also has the expectation that not everyone is going to come home, unlike a civilian airliner where the safety margins are much wider.