Amulet of Lost Voices is key to Larian's design philosophy. To account for player freedom, including the freedom to kill anyone, Larian need ways to let players know what's going on in the story that don't rely on any one NPC being alive. The Amulet solves that neatly. However, player freedom means that they're not guaranteed to find the amulet. So in places where the designers fear that players might murder plot points, they'll place a scroll of speak with dead in a nearby chest.
Swen Vincke calls it "n+1 design", and it does not apply to the Guidance necklace or the Resurrection scrolls. I don't think there's a deep design reason for their abundance. It's a balancing issue, in my book.
