A split-thickness graft takes only the upper skin layer, and the donor site usually heals within several days. The thinner the graft, the faster the donor site heals. Surgeons may even take additional thin grafts from healed sites. Donor sites are limited and autografting isn't always possible. Some patients can't withstand the additional trauma of a donor site wound.
Older patients heal slowly and have thinner skin to begin with.
And grafting creates another scar. Doctors often use temporary coverings while patients get stronger, or while donor sites heal for additional harvesting. Cadaver skin is preferable, but as with other donated organs, sometimes it's in short supply and transmission of infectious agents is a concern. In any case, the immune system rejects allo- and xenografts in a matter of days or weeks, and they must be removed and replaced. To avoid such problems, researchers and manufacturers are developing better wound dressings.