I just saw an image of a gecko stuck in amber 58 million years ago. Why don't animals rot when they are trapped in amber? I've seen dry insects whose chitin shells persist for a long time away from water and scavengers, but things with skin rot and dramatically change in appearance in the outside world.
things need to be exposed to bacteria to rot, amber produces a perfect form fitting seal and has several antimicrobial properties to boot killing off bacteria that is already present. Even microbes trapped in amber become preserved. This makes a lot of sense when you realize the resin evolved to seal injuries in trees if it did not prevent the spread of bacteria it would not work. HOwever preservation in amber is not perfect and the things do degrade, but do so so slowly that more geologically recently deposits have decayed so little is is almost undetectable, the type of resin also matters all resin is not equal and some amber resins preserves better than others.