I guess it depends. My mom's family ancestors have a cemetery they've used since the 1600's in CT, and it's still kept up. My dad's family are in a small, private neighborhood cemetery, about 100 years old, that's still kept up, mostly by family and volunteers, I think. I've visited several cemeteries over 100 years old, and they are still kept nice.
In another part of CT, very rural, there is a small cemetery in the woods, originally for smallpox victims. It got overgrown but a Boy Scout troop took over, cleaned it up, and now maintains it.
What do mean they are "knocked down and build over?" I didn't think that was legal unless the remains were reinterred elsewhere first. I know in the Charleston area whenever they find remains, such as unmarked slave graves found in a construction area, they respectfully disinter them and move them to another cemetery for burial. They never build or pave over them.