I know this is generally a network-wide policy, but I do not quite understand it.
Often discussion of an answer, especially about controversial parts, will help improve it. And, when such a discussion is then ultimately moved to chat (before or after it has reached a conclusion), it seems to inevitably die there as there appears to be a large subsection of site users who don't or won't use the chat system (which, to be fair, is horribly organised for discussing disparate topics such as comment migrations--being on the surface but one long thread, and all). I have rarely seen anything migrated to chat that had anything substantial added to it afterwards.
All that said, why are comments not for discussion and, by extension, why does the policy to move to chat exist?