The reason we bury bodies in coffins is to prevent decay of the body and to prevent odors and protect the environment from the decaying body as well.
Also the coffin is used to allow family to view the body and easily lower the body into the grave.
Although people will still rot in a coffin it just takes a bit longer and eventually the coffin unless it's stainless steel will either rust away or rot away in the ground and expose the body.
Coffins have long been used when burying bodies to prevent the odor of decay and to give the family members closure and prevent them from witnessing the decomposition of their loved ones, and in many cultures it has been seen as a necessary step for the deceased to enter the afterlife or to give back to the cycle of life.
Although in reality there's nothing harmful to the earth by allowing a body to rot into the ground because it's natural.