Why do we bury our dead (even when the body has been cremated)?

The Church has permitted cremation for Catholics since 1963, however, there is still a preference for burial of the body in a sacred place. Such arrangements are the most fitting for the human person, who we have faith will be raised from the dead on the last day. In 2016, the Congregation for the Doctrine of Faith issued instructions on burial and cremation in the document;
Ad Resurgendum Cum Christo (ARCC). Following the most ancient Christian tradition, the Church insistently recommends that the bodies of the deceased be buried in cemeteries or other sacred places… The Church who, as Mother, has accompanied the Christian during his earthly pilgrimage, offers to the Father, in Christ, the child of her grace, and she commits to the earth, in hope, the seed of the body that will rise in glory (ARCC 3).
The CDF added that cremation shouldn’t be chosen if the deceased indicated a preference for burial (ARCC 4). Cremains should be buried in a sacred place, such as a cemetery or a church set aside for that purpose (ARCC 5). Regarding certain “new ideas” on treatment of the human body, the CDF gave guidance:
You may not keep ashes in your house. Under most circumstances, “conservation of the ashes of the departed in a domestic residence is not permitted,” although permission can be granted by the local
ordinary “in grave and exceptional cases dependent on cultural conditions of a localized nature.”
You may not scatter them in the ocean, glue them into a necklace, or use them to fertilize a tree. “In order
that every appearance of pantheism, naturalism, or nihilism be avoided, it is not permitted to scatter the
ashes of the faithful departed in the air, on land, at sea or in some other way, nor may they be preserved in mementos, pieces of jewelry or other objects.”
(ARCC 6-7).
(Source: Catholic Answers)

Read more in the “Why Do We Do That?” series from Deacon Mike Fritz.