The reason that we do not use mail in voting is because it is too easy for election fraud to occur with an all mail in vote. There are some cases where people are allowed to mail their votes in, such as if they are out of the country, but there are checks in place to make sure that those votes are counted correctly and not duplicated or changed. To try to do this for every citizen of an entire state is logistically impossible, so a mail in revote would invite fraud.
Since the turmoil over the fact that Senator Gore came close to winning the Presidential race against Bush, there has been changes to the system in regards to how votes are counted. Recounts are often the focus on bogus voting machines and the mail in re-vote is not a solution.