I don't know if this is true or not, but what I recall from our yearly meccas to Bethel to visit family friends, we were told that the graves were intentionally not marked because they did not want anyone to use them as a shrine to the deceased. And then (again if I remember correctly) the example was given of Moses, Noah, I cannot exactly remember but it was a "prophet of old" and how they were buried in a quiet place so people could not use the site as a place of worship or them/their tombstone as an object of worship.
This may be total folklore in the JW faith, so I cannot confirm or deny the truth of what I was told.