The "reason" is probably that reply-to header munging is turned off by default in GNU Mailman and we continue to use that.
From the mailman 3 docs:
//The Mailman developers, and I believe the majority consensus is to do no reply-to munging, under several principles. Primarily, most reply-to munging is requested by people who do not have both a Reply and Reply All button on their mail reader. If you do not munge Reply-To, then these buttons will work properly, but if you munge the header, it is impossible for these buttons to work right, because both will reply to the list. This leads to unfortunate accidents where a private message is accidentally posted to the entire list.//
Older m/l etiquette (read 15 - 20 years back) were fairly strict about reply-to header munging, top posting, thread hijacking, excessive quoting, replying to the whole digest and so on :-)
Just my 2p.
-indranil