[Rune Son of Bjarne]

Of course it could be argued that the whole Rath Dinene/Silent street area is a massive complex, but it is still a relatively small portion of the dead who dwells here and only from the ruling class.

The same goes for the Rohiric mounds and even the Barrow-Downs. . .

I am quite sure that men just not turn into dust or fade away very quickly, so how do we explain this lack of graveyards?

I think it was just not important enough to make the books, if Tolkien was to describe every single aspect of every culture in Middle-Earth it would be too great a project. (and if then end result would probably not be the best read ever)