These diseases were the advanced civilization's fault of course, and they weren't able to stop it either from spreading. The last surviving group makes a frantic abandonment of the contaminated planet but succumbs anyway with their spaceship ending up being a Big Dumb Object that gets in everyones way. Or for something simpler, take an asteroid for example, they just float around space, but are still large enough for someone to waste their time fighting over what to name it. Even though all asteroids are big dumb objects, this is all too commonplace.