WUT?! Have you tried to look "shooter" section of the shop? You can find all sorts of FPS there from good old Doom and Red Neck Rampage to Crysis and the most recent Red Faction. Not to mention several incarnations of Unreal and Shadow Warrior and various hybrid like Deus Ex, Stalker and *cough* Postal 2 *cough*.

Don't forget shooters like Tyrian 2000 and Crimzon Clover. Also, Cuphead is here as well.

There's also Wasteland and Fallout, as well. (Although, in Wasteland, I've found that shooting isn't the best strategy in the early game.)