We're not trying to pretend that ground beef is the only food with germs living on it--there is, as we all know, an invisible film of bacteria living on virtually every surface on Earth. But the key word there is surface: the inside of muscle is largely sterile. When meat gets ground up at a processing plant, however, the outsides and insides of numerous animals get mixed together and mashed into homogenized meat spaghetti, and the bacteria is no longer restricted to the outside.
So while it's generally okay to eat your steak bloody--provided you've seared the outside to kill off the germs--burgers are a different story. The USDA recommends cooking ground beef until the internal temperature measures 160°F -- brownness alone is not a safe indicator.