There are cemeteries here and there, where you can be buried the old fashioned way, wrapped, un-embalmed, in a shroud in a shallow grave where you're worm food. The kind of grave where there's a mound above you that gradually sinks down to ground level. There's one in the country near my town. Cremation, while more energy-intensive, is still cheaper though.

The box is for the living. It provides "protection" to the rotting corpse. Nobody wants to get their suit dirty when being buried. Really its about that rational. 3000 $ later the dying part just got real expensive for the family. Even urns are overly expensive pots really.

Because a big paper bag would not handle the weight?

Because undertakers make vasts amounts of money selling all sorts of caskets and add ons!

I'd be for a Zoroastrian funeral I think it is where they take your corpse to a high peak and let the animals and birds have a feed.