well, let's forget about the silly religious nonsense for a second and ask a functional question: why do we dispose of dead bodies in such a wasteful fashion?

the reason is that we realized, apparently at some point very early on, that dead bodies can make us sick. we didn't come up with useful ways to understand this until the last century, but we realized that something about these dead bodies was causing us harm. so, we buried them, or we burned them.

even the zoroastrians, who insisted on feeding dead human bodies to wild animals, had the good sense to remove them from human contact by placing them on placards in the sky. a persian city in antiquity would have these fifty foot platforms with dead bodies on them, to be fed to eagles and vultures. gruesome, but arguably more holistic than cremation or burial.

now that we do understand this, why don't we adjust to it? because we also understand something else - all of that organic material is just going to waste inside of a metal coffin.