Because there is no reason for it. If you have a problem there is a contact email in every mail included. I do not feel like I should be sending mail from post

Because there is no reason for it. If you have a problem there is a contact email in every mail included. I do not feel like I should be sending mail from postmaster and the reply-to header does not feel any more appropriate. These emails are 100 percent automatic and do not require you to reply. Ever.