The reason that ground beef needs to be fully cooked before eating it is that fully cooking the ground beef kills bacteria in the meat that could make you sick.
If you don't fully cook the ground beef the bacteria could still be present in the ground beef which can then lead to food poisoning.
Also when the ground beef is ground up and when that happens any germs on the outside are stirred in and distributed evenly throughout the ground beef which can happen in the factory.
So there could be some germs alongside the bacteria as well that the cooking of the ground beef can kill.
So if you want to avoid possible food poisoning make sure to cook the ground beef fully to get rid of the germs and bacteria.