Other people exist in this space station/town-sized nuclear bunker/underwater city/sprawling industrial complex filled with slavering beasts, but you’ll never quite find yourself in the same room as them. While you can meet, talk, and even take objectives from them, they’ll always be just on the other side of a floor-length window or behind an impenetrable armoured curtain.
As a result, they’ll never come along to help you out, give you any equipment, or offer you the convenience of shooting them in the face if they’re the bad guy. Because that would involve the nigh-impossible feat of opening a door or walking through cloth. And if it is just a curtain you’re dealing with, you’ll lower your gun on instinct. Everyone knows that stuff is bulletproof. Pointless to shoot through it.