In recent years, the trend for shooters has been to move towards a multiplayer mode based model- increasingly, spearheaded by Halo 2 and the Call of Duty games, people buy shooters to play the multiplayer, with the campaign modes being ignored by most, and treated as afterthoughts by the few players who do play them. The culmination of this trend has been in modern shooters like Star Wars: Battlefront and Titanfall, which eschew single player campaigns entirely.
So it will probably interest you to know that shooter campaigns are extremely expensive- they can often single handedly be responsible for 75% of the overall budget of the game. That is according to Cliff Bleszinski, the creator of Gears of War with Epic, who is now working on his own upcoming game, Lawbreakers.
“[Campaigns in FPSs] usually cost 75% of the budget,” said Bleszinski to PCGamer in an interview. “And you burn through the campaign in a weekend, and then [players] go to multiplayer.” This, as well as the fact that they are such moneysinks, is the reason that developers are moving away from having single player campaigns in their games at all.
You can see the full interview Bleszinski had with PCGamer in the video below.