I've read some posts here on RPG Meta about comment deletion and it seems the overarching goals are to reduce clutter, diminish distracting information on the site, and maintain the focus of SE as strictly Q&A. This strategy is applied to both helpful comments ("You should add...") and to conversational comments ("this is so cool").
It appears from this policy that SE is not interested in providing a view into process. Questions and answers are regularly developed in a collaborative environment with plenty of back and forth between users in the comment section. However the historical value of this process is not appreciated in the tenets of SE. SE sees these comments as a disposable byproduct of its main function - a clear Q&A.
At the same time, however, SE does provide a view into question and answer edit history. Presumably, this is to provide transparency to the user about how a question or answer was edited.
Why does SE value transparency and the historical significance of edits, but does not value transparency and historical significance when it comes to comments?
I do not think visual clutter is a solid response here, because comments can easily be hidden, as they already are when there are more than ten or so of them, and as edit histories already are.