Eh, even if they were buried, you'd have to bury them more than 3 feet deep to really deter animals that'll dig up the corpse (like cougars and wild pigs). And even then, the conditions can be kind of fucky. Sometimes a body will get eaten up mere hours after you drop it off, sometimes the body will decompose in place and the remains will be there for decades until someone finds them.
Weren't most of those discovered via remains that he'd placed in his basement and inside his walls and shit like that? I'd reckon they're "still unidentified" because they simply became human soup due to decomposing slowly.
In case any other Anons were wondering, I only know about this because I've studied this subject out of curiosity, and I've gone on trails where I'd killed a roaming pig or dog and years later the remains (albeit skeletal) are still there. Shit, there's a full on deer skeleton that's 5 years or so old, literally just laying in an overgrown ditch by the side of a road near me.