FPS gaming, in my head, is sometimes the purest form of gaming. I play a lot of BF3 and it's me vs. the guy shooting me. My experience on the map with the kit and gun I'm using versus his.
AI can't replicate that experience.
I love other genres and games (Dragon Age: Origins is a game I'll never forget, I loved it so much) but FPS is such a reliable and solid genre. I played BF2 back when it first came out so I don't think it's just a phase. Nearly every other genre has flaws that ruin the experience a lot for me, long-term.