I thought the same thing, and I certainly see your point, but a controller has only a handful of buttons, where as a keyboard and mouse offer more options. Still in many respects a controller can be superior, I guess that's why many "pro gamers" use controllers on their gaming rigs.
They don't, actually. The ONLY FPS on the planet to use a controller for whatever reason is Call of Duty - and pro's are complaining left and right - especially after Black Ops 4 showed the world what CoD can do from a pro players perspective themselves via the blackout tournaments, which WERE HELD on PC - about CoD still being played with a damn controller.
If you're ever in the OpTic CoD scene, you would hear Karma (Damon Barlow, "pro" player; I love esports but it does get kinda silly vs. pro-athletes in Bball or Baseball like I used to play (not pro... lmao)) - constantly say "Why doesn't CoD just go to PC w/ mouse and KB, it's stilly" -
CSGO, Overwatch. Nothing they do on screen would be possible with a controller.
They're great for platforms and racing games wouldn't even be possibly by comparison with a mouse and keyboard, however. They're great all-rounders, for sure. And that's it.
Just have to say - this comes from a competitive perspective; I'm extremely competitive and that's where this argument and divide comes from - I can't STAND being held back by a controller, drives me up the wall and the controller in the floor. But if you're more chill and just wanna kick back, then sure, a controller works just fine. It is, however, indisputable which one is superior for FPS play. Not "hating" on consoles though, I love the PS2 more than most gaming setups and can't wait for next-gen. It will be, like dude said immeasurable performance for the dollar, monumental.