I think the coffin was nailed shut so that the corpse did not accidently fall out. That would not look good for those making a lot of money off of the dead person's family. After all, if the dead person fell out of the coffin somehow, do you think they would not blame the undertaker for that? And, next time they would take their dead somewhere else?
So like all things in life...follow the money and you will find the answer!
I believe the money spent on funerals would be better spent on the living. Especially the children. Just think, if we took all the money spent on death in America and used it for health care, every child would be taken care of!