If the old man in the Pardoner's Tale wants to die and looks like he is already dead, why is he not allowed to die?

Absolutely he is symbolic. He is not really an old man - he is the one whom the Revelers "attack" as being Death for which they are searching. They treat him badly; he then directs them to the gold under the tree which will be the cause of their death. He sends them to their Death. In a sense, he may be death since he sednd them that way, but they actually find their own death through their greed and their inability to be faithful to one another even though they said they would.