The .45 caliber bullet from the USP is plenty enough to blow the brains out of any zombie, thus killing them...again. While the Desert Eagle may produce more explosive killshots, the additional magazine capacity of the USP (almost double that of the Desert Eagle) makes it much more valuable in the zombie apocalypse. Plus, having less kick than the Desert Eagle, the USP will be able to dispatch of zombies more rapidly. It's all about accuracy, shooting the zombies in the head with as few bullets as possible, and the USP is ideal for that.