First off, they are moderated if the comments in question are flagged consistently. The system keeps track of the posts and comments flagged, too, so please flag the comments in question, not their parent posts.
Us moderators are basically users who volunteered for a tad more privileges than we used to have anyway to help the community, doing this beside all the other stuff life throws at us.
Therefore, we cannot possibly read all comment threads. We depend on flags bringing our attention to such things.
Also, at least for me it is undue abuse of power if I decide without any community reaction that I want to do something about a certain behaviour (as long as it's not a clear violation of basic rules) as I feel StackExchange should, before anything else, be managed by the powers and initiative of the community as far as possible.
Moderators, as I understand it, are failsaves who should only interfere if something goes terribly wrong and/or the powers of the community members do not suffice to deal with something disrupting their experience.
That is why I love this post and hate the lack of votes and reactions on meta in general.