It's possible that some cars receive proper burials, but which ones? Does Mater only dispose of the poor, sad cars who couldn't pay for a proper funeral? Mater does seem like a stickler when it comes to ensuring that he gets paid, reminding Lightning McQueen that he owes him $32,000 in towing fees. But even more weird than Mater being a remorseless capitalist about hauling his lifeless friends around in a post-apocalyptic automotive utopia is the fact that he's totally gonna cannibalize those friends for parts.
Look at the sign on the front of Mater's business: "Towing & Salvage."
The Walt Disney Company
If your car wife cannot be revived, you can rest easy in knowing that the star of Larry The Cable Guy: Health Inspector will be digging through her corpse to see if her break pads are still in decent shape so he can sell them. And since cars require constant maintenance, it's entirely possible that every "person" in the Cars universe is being powered somewhat by their dead friends' parts, recovered from a mountain of discarded corpses. That's Hell, right? I think that sounds a lot like Hell.