I'm very tempted to put the CoD and Halo hatred down to hipster-ish, sociologically driven causes, and that may be closer to the truth, but here's a more noble reason: CoD and Halo are recognized as awesome games to the exclusion of games that are equally as good but don't get nearly the attention. Hatred towards CoD and Halo may come as frustration at their disproportionate recognition.
That said, I love Halo, and have an on-and-off relationship with CoD. Halo's campaigns have always had a certain style to them, and they've all had consistently awesome art direction and music. Bungie is also very supportive of its community, and I have a lot of respect for them as a developer. Furthermore, Halo's multiplayer - when played as intended - is very cerebral. It feels so different than other first person shooters I've played that I've found the Dicta Bölke to be more useful than any squad based fire and maneuver or room clearing tactics. Not that it's hardly ever played as intended, but i digress.
Furthermore, CoD, while admittedly having its ups and downs, with considerably more downs than ups recently, has its moments. The Black Ops campaign, while short and with its flaws, was interesting, and had great visual direction. I don't enjoy its multiplayer too much except for quick games with my friends, but it's not rage inducing. Of course, pretty much everything about MW2 sucked except for the Spec Ops, but WaW and CoD 4 were both pretty awesome in my opinion, CoD 4 undeniably so. I still play both from time to time, after all.