BoM/TWS Message Forum

From "Magic: The Libra Personae" : "Setting the earth on fire fuses the magician's essentia into the golem in a way that also burns out any [semi-decayed] anima.

If you're asking for in-world explanations, I guess it takes a long time to burn through that much dirt, and as the fuel runs out it can sometimes snuff itself out. In this, the magic mimics the sort of behavior that happens in real life. Think how much time it can take install an operating system, and how fires can snuff themselves out and need to be relit.

If you're asking why the author invented such behavior, the first is a delaying tactic so that there's room for complicating developments to be inserted into the story. The second is just to fuck around with the characters' heads.