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The biodegradability of people, mainly. The sacrum and mandible last longer than other bones, but unless they're in the desert or something crazy happens they'll go away as well after a few years to decades.

Semi related: In New Orleans they have these above ground graves that decompose bodies pretty quickly. Generally there's not much left after a year of sitting in an oven over a New Orleans summer. Back in the day they'd bury them for a year and a day, pull out any metal bits remaining from the casket they used, then shove the bones back in where they would fall into the caveau to further decompose. You could have dozens of family members sharing a single burial plot.

We do love slow cooking things down south.

Edit: I didn't mention that there are some remains still sticking around. Look at the other responses for more of the story. I just wanted to throw out some facts to set up my slow cooker joke.