Okay, here's a thing you might not be thinking about: Context.
First Person Shooters have a fairly limited range of contexts wherein their primary interaction with the gameworld, shooting, is appropriate.
Therefore, they will tend to be forced to tread those paths quite well. War or armed resistance is a common feature, whether against other people or against weird gribbly aliens, because there is a fair amount of shooting to be done in war.
Gangland shenanigans are an occasional feature, because these people likewise have a tendency to get a bit shooty with one another.
Beyond that. Not much. There aren't many other contexts within which shooting stuff works as a primary gameplay mechanic, so FPS games don't tend to explore them.