Perhaps the same reason Farzad did. Both artists were probably hired to work for OWI for a specified amount of time. For Steven Olds, it may have been from 1994-1997, since that was the time period AO was made. For Farzad, it was probably from the beginning of MO to the release of the game. Unlike Raymond Swanland, who worked on many different subjects (outside of Oddworld) at one time.