Bacteria collect on the surface of meat. If you're cooking a rare steak, even though the internal temperature may only rise to 125 degrees or so, there's not as much of a concern because the surfaces of the meat typically reach much higher temperatures (i.e. when you sear them for a quick minute). When you grind a piece of meat, that bacteria gets distributed all throughout. That bacteria may now be living smack dab in the middle of your hamburger patty. That's why it's important to cook ground beef to a higher temperature than you would a steak.