<p>This is more of a comment but it grew over-long.</p>

<p>If the question is "why should such a policy exist", I can only say it's part of the original site design (so I guess, ask Jeff Atwood and Joel Spolsky what their reasons were and what evidence they might have considered for making it that way, though they have written a number of posts that give some hint of what things they were trying to <em>avoid</em> building - on their personal blogs, in the stackoverflow blog in <a href="https://stackoverflow.blog/2008/page/17/">2008</a> and 2009, and in other places) and that a network-wide policy discussion probably belongs on the network meta. </p>

<p>Edit: Alecos has explained that the remainder is not relevant to the intended question. I'll leave it here for the present, however.</p>

<p>If the question is "does a formal policy exist" you can find numerous references to such a policy. For example</p>

<ol>
<li><p>In relation to the present instance, see <a href="https://stats.stackexchange.com/help/privileges/comment">https://stats.stackexchange.com/help/privileges/comment</a> see especially "When shouldn't I comment?", in particular the item, "<strong>Secondary discussion</strong> or debating a controversial point". </p></li>
<li><p>The FAQ, specifically <a href="https://meta.stackexchange.com/questions/19756/how-do-comments-work">How do comments work?</a> - particularly "when should comments be deleted?" and "Miscellaneous information" in the answer</p></li>
</ol>

<p>But before we even need to go and find these specific words elsewhere, the fact that Stack Exchange got someone to code a tool to auto-move messages to chat and which posts that <em>not-for-extended-discussion</em> comment itself suggests that it is official policy to do so (why would they spend money to provide mods on every site on the network with a tool to do something that was not a mod behavior they considered important behavior to encourage?) </p>

<p>While we're more tolerant on SE of informative comments that are not-quite-answers on our site than many other network sites (it's part of the nature of the topic, I think, though you see it to some extent on math.SE and mathoverflow for example), nevertheless comments are primarily designed for improving a post and in that role, are generally intended to be temporary. </p>

<p><em>Extended discussion</em> formally belongs on chat, by design.</p>
