OK... a possible "in universe" answer that corresponds very closely with what
@Eljorahir posted... except the universe is not only the story's universe; it's the entire occult/supernatural universe of the cursed/damned .
So , first let me make an analogy
** ...
Remember the question that always comes up about LOTR (but is typically asked by those who have never read the
books), to wit: Why didn't one of the
Eagles just take
the Ring and drop it into the cone (the top) of Mount Doom?
Ans: because ME is governed (like all
realms) by
a Ruler (or Lawgiver).
And the Lawgiver said: "Nope... can't do that, at least not this time and in this situation." And that was that.
So likewise (
mutadis mutandis) the damned world/
realm is also governed by laws, however twisted and perverted they may be.
One of the laws may be that the dead, when formerly involved, willingly or no (as Gregory Peck's infant son was), in infernal rites and practices, have to be buried in separate graves in profaned graveyards.
I don't pretend to know those rules, nor do I have any desire to become acquainted with them in the least.
Just saying that the real life Wiccans in our times have their rules and rites, much as the Druids did more than 2000 years ago.
So it makes sense that in a story about the supernatural and the damned there there are going to be associated rules an rites that have to be observed, even if they are not understood, or hidden, from mere mortals.
Just my 2 cents.
** For those who read my responses in other threads... I'm building a clock, OK? 😉