If you were to just throw it all over the place, then there would be radioactive waste all over the world and people would get sick, not to mention the damage it'd do to the environment. By keeping it all in one place, they can contain it and prevent it from harming the environment (well, at least the rest of the environment) and people.
6.4 poison exposures/1000 population, 37.4 poison exposures in children younger than 6 years/1000 children, 1 poison exposure reported to U.S. poison control centers every 15 seconds.
