Game companies can make games in any setting they want, and yet some of the triple-A companies would rather jump on the bandwagon that's rolling for shooters these days called the modern-day setting.
This is a setting that takes place from the 1990's to the 2010's (the time we live in currently). There we have guns like the M16 and the M4, vehicles such as the M1A2 Abrams and the AH-64 Apache.
Unfortunately, too many shooters have been using this setting to rake in sales from the players that like to follow fads and go with the flow instead of thinking outside the box. Call of Duty, Battlefield, Medal of Honor are some of these shooters.
There are other settings that have been barely scratched on the surface. The Vietnam War, the Korean War, World War One, the Civil War, the future, etc. Yet, they're still like this today.
As long as people keep buying games in modern-day settings, we'll never get to experience the wars and eras before or after the time we live in on shooters. Other genres have taken the initiative to explore other settings (RTS, MMO, etc.), so why shouldn't shooters?