I was driving past my towns cemetery last night just as the sun was setting, and it brought to mind a number of books I have read...
So in these books the necropolis is filled with undead but they always come bursting from the ground. So it comes to mind that undead (with the exception of Vampires) don't need to sleep/rest... And most have no need for feeding and such.
So why do the masters of undead keep them in their graves? Is it to keep up their moral which they don't have?