I guess you just have to play them to understand.
i have. 
I mean, getting deep into the game. Playing around with Forge in Halo 3 with three other friends, or getting a really good party going and playing an insane game of Rocket Race (all two-man teams, so it's especially funny when someone in the party is paired with someone they don't know). Or in Call of Duty 4, finding a really good Hardcore match, where people communicate well and teamwork is really there. Not to mention all the unlockables and challenges in both games.

The games are decent on the surface, but the way you dig away the coal to get at the diamonds is by getting really immersed.
Like SA said, they don't do a lot that's new (but there are things there, for certain), but what they DO do is done perfectly. The games aren't revolutionary... they're just damn fun.

And the developers' support is constant, so the fun is just going to keep coming.