The problem with keyboards for multiplayer is that many old and/or cheap keyboards only allow for 3-4 keys to be pushed at once, which makes it very hard for there to be more than 1 player at the computer (or even one, in some games).
Oddly, it's something that cropped up on PCs only whereas other platforms from the 80s and 90s allow for far more keys to be pushed. I've a ton of old RISC OS games that are 2-player (or even, in one case, 8-player) on a single keyboard.
Oh, talking of those old games, a port from RISC OS to PC and Linux of Spheres of Chaos:
http://www.spheresofchaos.com/Kind of like Asteroids on LSD. Lot of fun. And that's the 8-player one I was thinking of as well. One player can be on mouse, but it's hard to control.