Because in the US almost all people are required by law to be embalmed... with a toxic fluid called formaldehyde. The funeral industry is a big racket. They made it required to have a funeral parlor embalm people whether they want it or not (unless immolation is allowed). And you can no longer have a visitation, or "wake", in your own home. And some jurisdictions required grave vaults (my grandparents). The only reason I could think of was so the formaldehyde will not get into the ground water.  If formaldehyde was not used, there would be no reason for vaults. See the moneymaker here?  Most people have no clue how poisoned their ground water is, but most ground water is poisoned at least a little. It's hard to escape it.