Most cemeteries in Europe have been destroyed to make space for urbanisation. The underground of Paris is full of bones from those cemetaries (catacombs of Paris). It's fairly common as outside of a few aristocratic families, people keep a very short memory of their ancestry. I have a very vague knowledge of who my ancestors were back to the mid XIX century and nothing prior. Hard to have any emotional attachment to people you never heard about. So if no one cares, better to build a parking lot!
I've spent some time on genealogy, and really beyond grandparents and maybe great grandparents it's mostly curiosity and for the interesting stories, and I only care about the graves to the extent that it makes solving the puzzles easier. It's not like there is anyone alive in my family who knew older ancestors of mine than my great grandparents.
For my part I've only ever seen even my dads grave once, and feel no need to go back there, though I understand why some might feel it matters to have the grave of someone they knew in person because of the emotional attachment.
Some cemeteries in London reuse graves after just 75 years
https://en.wikipedia.org/wiki/City_of_London_Cemetery_and_Cr...
I have very vivid memories as a child of walking through Brookwood Cemetary in south east England seeing a big machine crunching up headstones. Vast swathes of the place was flattened. The cheat code here seems to be "topple testing" for health and safety reasons.
It's the UKs largest cemetery and in a shocking state:
https://lostlcp.com/brookwood-cemetery-reburials/