These two video games are similar to each other, but their similarity to themselves is what has drawn attention to their creators. What changes from the first Call of Duty or Halo to the next? And what happens to the next in the series to distinguish it from the earlier versions? Sure there is better interface graphics and user response within the game, but these aspects are technological feats that get updated every time a new game is released thanks to the constantly improving electronics and game consoles. I believe that nothing actually changes from title to title of these franchise games, and this is what publishers of these video games are purposely doing.
Activision and Bungie are the video game publishers responsible for the releases of Halo and Call of Duty, respectively, and they focus on putting up a product that pleases the costumer. This seems like the right way to produce video games, but the problem lies in the static nature of their releases. The first in each franchise was well accepted by video gamers, so well accepted, in fact, that Activision and Bungie were compelled to make more and more titles of the series. These publishers know that their audience already exists, and they aren't willing to take chances with losing revenue by introducing new aspects of the games that could possibly displease their target audience. Because these publishers believe this, the unique aspects from game to game in the franchises is very limited. This is unfortunate for new video game developers with exciting, new ideas because their ideas will get shot down for, say, the next in a successful series: Halo 5. The next time you consider buying a new video game, rather than looking at the n-th continuation of a series, look for new games with more interesting stories or game play for a different and most likely entertaining experience.