Dude back in 96-98 all FPS had co-op. All build games( blood,Duke3d, shadow warrior) Hertic, Doom, most Quake enigne games ect,ect had co-op, matter of fact I cant think of any big name FPS that didnt have it back then.
That's because AI in those days was: See player. Run at player. Scripted AI and squad based AI, starting with Half Life (and continued by most games since then) is way
harder to get squads and scripted actors to respond convincingly to more than one player. They all either head off to hit the first person they see, or the middle one in the group, etc. It's hard enough programming a squad to react to one player, his movements, cover, weapons etc.
Admittedly, games like HL2 (and a bunch of others) have you fighting with ally NPCs and so on, but they themselves are AI controlled and aren't likely to act as unpredictably as players.
Serious Sam, on the other hand, relies (ok, relied - I haven't played the new one) on fairly simplistic AI made up for by loads going on at once. Don't get me wrong, that's not a criticism (I loved the first one) but those exploding screaming kamikazes are the definition of "see player, run at player." Thus, arcade style games like SS are more easily adapted to co-operative.