Second, God created man body and soul. We are bipartite creatures. And I think lovingly tucking the body away in a "casket" (definition: a container for treasured things) shows more respect that burning something up.
Third, burial is a metaphor. Just as we are baptized under the water and brought up as a picture of Christ's death and resurrection, so our burial is a picture of us "sleeping" and waiting for the resurrection . . . and there's nothing metaphorical about that.
Burial conveys the image of sleep, the metaphor Jesus and his apostles used repeatedly for the believing dead (John 11:11; 1 Cor. 15:51; 1 Thess. 4:13–14). It conveys a message, a message quite different from that of a body already speed-decayed, a body consumed by fire.