Netcode, gun feel, balancing, map design, sound design, ... all things that are present in co-op shooters as well. Don't get me wrong, I agree with what you're saying, but I feel like you have misunderstood what I was trying to communicate. (Which might be my fault.)

And yes, there are things that are unique (or more critical) to PvP shooters, but my point was: It's overall less work, for developers and artists, to just have players fight each other over and over again, than to create content for players to cooperatively enjoy.