The most major difference between CoD and Mario games is the amount of games. There have been 19 mario platformers in 30 years, and that is including 3d games and games on handheld and consoles (not re-releases though). For CoD there are 10 games in 11 years, only including main series games (21 if you are including handheld games and "expansion" games).
You can see why one would seem more "milked" than others, but if it sells and people like the game, then why not?