Can someone please explain to me why the hell Halo has the same rating as GTA, Half-Life 2, and Doom 3? It just seems completely implausible.
Because it has people being shot in a relatively realistic manner, probably. Even if not shot by you......
It's because the US doesn't, again, have a proper ratings system. In the UK, Halo is 16+ (under PEGI guidelines, which I think have been mostly dropped for the BBFC style U, PG, 15, 18 style; really it's about 15 rated) and GTA is 18. Halo is 16+ for depicting 'graphic violence towards unrealistic human or fantasy characters', to paraphrase. Call of Duty, for example, is 15 in contrast (not so graphic violence, I presume, because there's less by way of blood decals etc), as is Half Life 2 (possibly, I guess, because it shows unrealistic humans etc). I suspect Halo would be 15 rated if they'd used the other (BBFC) code rather than PEGI.
Anyways, it's because the US has a ****ed up system that meshes the equivalent to UK 15 & 18 ratings into a single 17+ one, because 17+ is overly high for it's content, and AO has an undeserved stigma akin to porno, both I'd envisage because the US (or at least its legislators) has become so prudish.