If you were Infinity Ward/Treyarch (or Activision, who owns the CoD License), and you knew that 200,000+ players per day were playing the same game each year, on sometimes a new console (From PC, to Xbox, to Xbox360, PS2, PS3, Wii etc) you would want to pump out yearly titles as well, its a business standpoint.
Look at the hype around any CoD game, CoD5 sold off the "Includes CoD4's Engine" hype alone. Infinity Ward did create that engine, therefore fanboys emerged, and multiplied daily. COD4 was an instant success, so why not tease if they have nothing to show yet? Hype sells games these days.