Aside from “that’s the way it is because that’s the way it is”, one of the largest reasons is probably that the developers don’t want to encourage or reward players for being zombies, either because they intend the game to be human-centric (and thus being a zombie is a supposed to be a bad and unrewarding thing), or because they fear it would unbalance the game. Thus, you wouldn’t get rewarded exp for doing zombie stuff.
You would think that if that’s the case, brains shouldn’t give exp either. But brains are the primary thing you need to redeem (become human again), and thus you would be rewarded for getting them, because that means you’re working towards a good thing.
Of course, there’s a option to disable redeeming, which kinda circumvents the whole system. If the devs really wanted to drive the point home, they should remove exp for brains and instead give exp when you redeem.