Why? Excellent question. You should always ask that question, even if you think you already know the answer. You will find that a great many things are far deeper than they appear.
Madness string 4384
The flames invisible, the poison intangible? The fires that feast on both flesh and bone? The flames that remain unquenched, never-sated by their thirst for the death of all things living? The heat that lurks, forever waiting? Or, like, some other fires?