There are laws in Maryland (where this particular cemetery is located) which forbid building on a known cemetery site - or that require the developer to bear the expense of exhumation and reburial elsewhere.
This, of course, is seen as a big problem for a developer who wants to maximize his profits. So, they try to minimize the cost in any way they can and still develop and build.
Many pieces of property have exclusions in the deed which state that a cemetery is located in a certain place on the property, and some even have "right of way" paths platted out for relatives to visit the cemetery. However, the areas marked out usually only include cemeteries of the original White owners which have marked graves, or maybe a fence of some sort to establish a known boundary. Burial on the home farm or plantation was common place in the 1700's and even into the 20th century.
Servants of those farms (mostly black people) were usually buried in unmarked graves very close to, but separate from the family cemetery. So their graves are very often NOT accounted for in land deeds.
The sad thing is that these unmarked and unrecorded graves are easily located without the need to disturb the ground in any way. I personally do this all the time.
But again, developers do not want to find any graves and it is usually only when a grave is disturbed and reported that work comes to a halt in their development. They then will often hire an "archaeologist" to dig some shovel holes and declare that there is "nothing there" so that work can resume.