Not sure that you want to know the answer to this!
Basically bacteria and fungi in your body start tyo use you as food – you then become basically the hard-to-digest bits (bones)
I once heard a talk where the body was sealed in a lead coffin – completely sealed. It was dug up by an archeologist after 100 years and there was a “soup” in it as the rotten/digested body bits couldn’t drain away – yuk!
I think part of the reason is when we die, our bodies no longer protect themselves form bacteria and other small bugs that will eat us up! Some bodies don’t decompose, mummy’s that have been preserved, and certain other people throughout history.
What a good question though, I don’t think I really know the answer!