Fortunately, the concentration of preservatives, which are sugar and so, in the body does not harm the microbiome of the gastrointestinal tract, for example. Otherwise you and I could be seriously hurt.
Salt and sugar are not preservatives AT ALL, but only in certain concentrations. For salt, this is usually a few percent (although there are exceptions - for corned beef or cold-salted mushrooms - there are already more than 10%). For sugar, this is tens of percent.
In the body, neither sodium chloride nor sucrose can reach such a concentration. If they could, they would have conserved not only microbes and fungi, but also the human body itself.
And the corpse doesn't care whether microbes and fungi eat it or not))