When you submit your ballot at the machine, its at the point of counting and should have no time or chance for other people to even touch it until its counted.

When you submit your ballot via mail, that ballot enters the mail system and follows a procedure that involves other people with access to manipulation or removal. Its really not that damn hard to figure out.

What path does the ballot take? If it isn't directly to a point where nobody but the submitter touches it, then its already insecure. That's how security works.