Because it will be a war that will never end. The real fix is to improve mail clients so that they understand long standard List-* headers and make it easy for their users to do the right thing.
It's true that Reply-To munging may be useful for some lists, but the use cases are *very* limited, such as an announce-only list that redirects to a discussion list. Launchpad's mailing lists are primarily intended for discussion lists, and reply-to munging is generally not appropriate in those cases.
Because it will be a war that will never end. The real fix is to improve mail clients so that they understand long standard List-* headers and make it easy for their users to do the right thing.
It's true that Reply-To munging may be useful for some lists, but the use cases are *very* limited, such as an announce-only list that redirects to a discussion list. Launchpad's mailing lists are primarily intended for discussion lists, and reply-to munging is generally not appropriate in those cases.