Why can't dead people be revived?

Technically, dead people can be revived, if they've been dead for only a few minutes. This is called clinical death, and it can be reversed. But for people who've been dead for over a week, then probably not.

First of all, when people are dead, they immediately start to rot. They will start disintegrating to smaller forms. When that happens, a horrible smell will start to rise, then eventually be disintegrated to the point they they are only skeletons. If someone dead has been left in the open for a week, then it will be a skeleton in only about that long.

This is the main reason why someone dead can't be revived. If someone is brain-dead, but is not clinically dead, then you can't revive that person as well, because their brain is not functioning/responding.