Graves aren't for the dead. They're for the loved ones the dead leave behind them. Once those loved ones have gone, once all the lives that have touched the occupant of any given grave had ended, then the grave's purpose was fulfilled and ended. I suppose if you looked at it that way, one might as well decorate one's grave with an enormous statue or a giant temple. It gave people something to talk about, at least. Although, following that logic, I would need to have a roller coaster, or maybe a Tilt-A-Whirl constructed over my own grave when I died. Then even after my loved ones had moved on, people could keep having fun for years and years. Of course, I'd need a slightly larger plot.