I have never been to a funeral here in central PA where the grave did not have a vault/liner. As someone else said, I believe many cemeteries require them so the ground does not sink when the casket finally succumbs to deterioration. Saves them the cost and effort of going around filling in sunken areas and re-seeding.

Can the extra dirt & reseeding really cost more than the cost of the vault?

The cemetery pays for the fill, seed, and labor, they don't pay for the vault.

There it is.

That coffin won't last long without a concrete vault. I've never seen that done before.

The coffin only needs to last until they're done filling the hole. If you want your body to last forever, build a pyramid.

Or get mummified.

Now accepting contract bids and slave labor applications for my burial pyramid.