People play games with dance pads, or with their feet, or with Guitar Hero instruments, and they have fun doing it, but that doesn't mean it's efficient, or optimal, or that those control schemes are "good" for the games they're using. If your argument is "Controllers are adequate for FPS games", sure - I don't think anyone is refuting that point, but that's not the argument you made - you stated that controllers are good for FPS games, which is a pretty subjective word, but clearly you posted in the right place, because as you can see from all of the replies here, it's a very unpopular opinion.

Clearly people can play FPS games with controllers. It is an option a lot of people use. However, it's just the case that an equally skilled player on a controller will lose to a similarly skilled player on M+K. There've been numerous attempts at scientific tests to prove this. Here's one such example. There's an anecdote that years ago, Microsoft was considering offering cross-platform multiplayer between PC and Xbox, but scrapped the idea when it was discovered that very skilled Halo players using a controller were losing to objectively less skilled players using mouse + keyboard.