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.
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.