In many areas, even six feet under, we will have shifting and movement of the ground. couple that with a large rain event and we have freshly buried remains shifting in the soil, even becoming exposed. In most areas even those buried in simple cardboard or shrouds require concrete vaults to keep the body in place.
Cremation becomes more and more appealing to those without a religious tradition.
I haven’t heard of bodies surfacing except in flood zones.
I’ve heard of the wood casket collapsing and a depression forming in the soil, but you can just top it off with more soil and plant grass.
My family cemetery is not in a flood zone, but the state requires a concrete vault. All I want I a pine box and no vault.
I don’t think funeral homes offer the plain pine box. Nope, it’s particle board - tacky, icky, awful. So people upgrade. Most people, anyway, my mother stuck to her guns and bought the particle board coffin for her mother.