I get killed as a gunner, so the pilot is dead too?

[Nil]

"not realistic" is only one of the argument, and of course anybody can make fun and troll about it like you did because it is just a simulator, or a game, whatever you call it.
But I am serious, this is not a topic for entertaining people, it is about a real issue.
One of the other reason is to not waste our real time life which is not virtual but real.
It is not fun to have a gunner killed and not be able to continue flying even if the pilot is still alive.
In some single player games, you can take control of IA, like IL2 Cliff of Dover (I believe but not sure), B17 and others, which allows us to use our precious time life more in a effective way and not waste our time to hit fly again, take off , navigate...
I which I can create my own simulator , listen the community, take their feedback, modify the sim ... but that is an another story