Fossilization is rare because of this, something like less than 1% of animals that ever lived we think made it to fossils. There's quite a few ways something can be buried quickly and avoid decomposition. Bogs and low oxygen water bodies are great at preserving as things rarely decay inside and scavengers can't get them. Mudslides and quicksand can bury and animal rapidly. In the ocean if a carcass sinks and is buried faster than microbes can break down what's left they can be preserved. Deserts are great at preserving as they are dry, so long as again the sand buries them before any scavengers majorly damage the carcass. Lots of small things fossilize much easier too. Bones of animals that have already been eaten and then excreted by an animal can survive as the poop can be fossilized. Many shell making animals are essentially pre-fossilized and so their shells can accrue in things like beaches and basins. Tree sap is also great at making fossils as amber as it can be near instantaneous.