For one thing, of course, the evidence is often consumed by the crime. For another, insurance companies may bemoan their arson losses after so many years of blithely collecting payments, but the fact is that an arson investigation is more expensive than it is worth to the company's bottom line. It is cheaper to write it off and go back to collecting payments.
Of course, there is always some dimwit who, as I once saw in a post-fire folder, claims an entire room and its furnishings when the room was not part of the extant structure.
There are a few tips for the would-be arsonist (don't forget the roof, for example, make sure no one is at home, etc.), but otherwise arson is not a bad gig.
It wouldn't surprise me a bit if arson claims began to rise in these parlous times.