In the old days, political operatives would have to have physical access to a ballot box in order to stuff it with fraudulent ballots. Today, political operatives can simply use the U.S. Postal Service to do the stuffing for them.
How would you feel if you exercised your legitimate and constitutional right to vote for your preferred candidate and you later found out that your neighbor had mailed in a ballot for his dead mother, father or spouse for the opposing candidate, thereby nullifying your vote?
And there is a huge difference between absentee ballots and mass-mailed ballots. Absentee ballots are requested, and your signature must be witnessed. Mass ballots are mailed to every registered voter, whether they are dead, have moved or are simply the family dog.
Third world countries do not allow mail in voting, why should we?