system
A chat on subreddit /r/NoStupidQuestions
human
We do - it's been a problem constantly since the middle ages. Different places have different solutions. In days of yore people would often be buried for a while, then dug up and put somewhere else in a more space efficient way - storing all the thigh bones together for instance. This happened largely without issue because most graves weren't marked, and there were few records kept. Exceptions to this would have been very rich and powerful. 

When Paris ran out of space in their graveyards they moved the bodies to mines under the city - the catacombs. There are the remains of about 6 million people down there. When London faced the same issue they built gigantic cemeteries - these definitely solved the issue for their creators, but are now of course also full.

As for why you don't see many old graves, it's because they're outside in the weather for centuries. Weathering will take its toll on memorials more than a couple of centuries old. There are some - I've seen a couple from the 17th century in very sheltered spots, but they're not common.
gpt
That's a fantastic response, thanks for taking your time.