Those crowds of people is asscreed are (essentially) mindless drones though, just milling about with no real intelligence other than "walk to here, run away if a fight breaks out". Same with the serious sam goons, most aren't smarter than "Run at sam while shooting". having that many enemies onscreen while also having them behave and move in an intelligent manner would be much more difficult.
now, that being said, we haven't really seen the Doom 4 monsters act all that smart...