It's not for the dead. It's for the survivors. Many people want a place to gather, pay their respects, and create a tangible connection to their loved ones. There's no theoretic reason why it has to be an individual headstone (versus more dense monuments) or even an outdoor headstone setting (versus cremains in an urn or scattered in a meaningful place) but don't pretend that it has anything to do with the dead. It's a ritual for the living.
> don't pretend that it has anything to do with the dead
It is for the dead. No one is visiting graves of someone who died 200 years ago, yet everyone is up in arms if you mention moving an old cemetery. If you own farmland with a family cemetery (again, 2-300 years old), you have to farm around it, you can not move it.
Perhaps they're special cases, but I've visited cemeteries of people who died in the 1600s and 1700s in Boston, Plymouth and Salem, Massachusetts. You'll find two cemeteries almost across the street from each other in Boston, right in the center of the city. They're tourist attractions, both for the now-obsolete grave decorations, and for some of the famous people who are buried or at least commemorated there. For instance, you can see markers for people who died in the Boston Massacre in 1770 in the Old Granary Burying Ground; needless to say, it's named for a building which used to stand nearby. King's Chapel Burying Ground, just down the street, has a bonus historical feature - a ventilation shaft for the subway, which I think dates to roughly 1900.
They are special cases. Very, very special. There is a 150 year old family cemetery a stones throw from my house. No one cares. A quarter mile from my last house there was another one. I drove past several on my way to work. NO ONE CARES. The number of old cemeteries that are of any interest is so minuscule compared to the number of cemeteries.
Cemeteries are a growth of the Christian idea that you're going to be resurrected one day and peoples general fear of dying and as long as someone knows where they are they're not really dead. They are a colossal waste of space.
Moving the bodies themselves seems to be an issue, and I expect there would be awkwardness about allowing farming where there had been a cemetery for perceived health reasons, but there have certainly been cases of repurposing cemeteries. Pioneer Park in San Diego, while having a scan few recent articles about how it was probably a bad idea, didn't seem to cause a huge controversy: the city took an old cemetery in disrepair, took out most of the markers, put up a little memorial in the corner, and turned the rest into parkland. The primary school right next to it, at least when I went there, even used it occasionally for classes, though I expect that has stopped for perceived security reasons.
There is a truly ludicrous amount of unused/underused land left in the US. The only problem is where we are burying people, not that we are burying people in the general case. Cemeteries in cities/suburbs get in the way, but otherwise it isn't a real concern.
Seriously, drive across Wyoming sometime and then try to say with a straight face that we are running out of land.