The main reason? They're fun and people like playing them.
There are secondary reasons and no, it's not about guns, it's about conflict, which is the core of video-games in general. There has to be a way to resolve conflict and defeat the enemy. That often involves projectiles of some sort, be it guns or some type of superhuman power. Many games which feature shooting as the primary method of resolving conflict also have other mechanisms, like the powers in BioShock Infinite or Borderlands.
Most games come down to either shooting something or hitting it directly. Mario kills enemies by jumping on them, Link kills them by slicing them with a sword. Games like Skyrim feature both ranged and melee combat.
Of course, it's also obvious that "shooter" isn't a genre. The Last of Us is a different type of game from Call of Duty, which is different from Fallout, which is different from The Order, which is different from Deus Ex, which is different from Mass Effect.
As far as diversity goes, the modern era has so much more variety than the age of platformers and beat 'em ups that it's not even funny.
