Lore reason: People are too cheap to buy anything but default death stones.
Tinfoil hat: They were all built by aliens!The purpose of adventure is to shine light into dark places,
Poke monsters with a sharp stick, Then steal anything that isn't nailed down!
To the Manor Born QFC 185-186-367-65788716
While, yeah, it probably is just asset reuse, I came up with a theory some time ago that Eastern Occultism and Western Ritual Magic are in fact branches of the same discipline, due to their similarities, connection to demonology, and, specifically, how they draw on life essence.
Ritual Circles use Mahjarrat characters, implying they may have originated with the Mahjarrat - or maybe just due to Zamorakian predilections. Anyways, it's a bit out there, but my headcanon was that a Mahjarrat (originally Temekel, though I chose him arbitrarily) created Ritual Magic for use by humans, fled to the east, and founded the Occult.
At any rate, the point is, if they DID want to give those gravestones some significance, the lore is open enough, and enough indicators exist, that they could totally make the headstones a Mahjarrat design.