I mean, as an analogy think of plastic bottles. Left alone they break into tiny tiny pieces that get everywhere. This is often caused by the sun or abrasion, but underground you've got the preparation materials, rotting flesh gasses, and plenty of microbes.

I would imagine that any artificial thing (including implants) could last forever when left undisturbed for any given length of time

Then you have a fundamental misunderstanding of how things happen, and an oversized confidence in modern caskets.

Not to mention what seems to be a complete lack of understanding what "artificial" means