Fossilization is rare, and I’m of the view that death by rapid burial (as with the Burgess Shale, or death assemblages such as have been found in volcanic strata) or burial not long after death are generally the best explanations. OTOH, in Pompeii there are plaster of Paris casts made inside cavities left when buried victims’ remains decomposed — no bones. So, even that doesn’t happen every time.
Rapid burial by volcanic ash is rather easily comprehended; by other means, not so much.
The massive, incomprehensibly rapid burials of trilobites comes to mind, preserving gazillions of their remains intact over wide areas, their structures amazingly intact, even in some instances their chitinious exoskeletons not mineralized, while fossil remains of their evolutionary predecessors elude us, denying us clues as to the developmental steps leading to the amazingly (and that word fails to render descriptive justice) complex trilobite eye .