Yes I know the purpose of the comment field, so no need to lecture me on that.
I would disagree that this is the case. The question your asking here is proof of that.
Comments are intentionally disposable. From How do comments work?:
Comments exist so that users can talk about questions and answers
without posting new answers that do not actually answer their parent
questions. Comments are often used to ask for clarification on,
suggest corrections to and provide meta-information about posts.
Comments are intentionally short, having maximum length of 600
characters, and allow only limited markup. URLs in comments
automatically become hyperlinks. Each user may post only one comment
every 15 seconds.
Comments are disposable: unlike posts, there's no revision history,
and they can be deleted without warning by their authors, by
moderators, and in response to flags.
As for why your comments are disappearing, if they weren't deleted by you, they have been flagged by other users or a moderator. Again, from that meta post:
Comments that are flagged by multiple users are deleted automatically.
The number of flags needed is based on the comment's score. It
currently takes "3 + (Score / 3)" flags to delete a comment.
Moderators can delete any comment, or purge all comments from a post.
Specifically, from that meta post, I draw your attention to:
You should not expect them to be around forever: Once a clarification
has been made, an edit added to the post to include new information,
or the issue in the comment is otherwise resolved, it is subject to
deletion. In reality, many obsolete or chatty comments remain
untouched due to the high volume of comments posted, but this does not
mean that they can't or shouldn't be deleted in the future.
I would characterize your commenting behaviour on the site as being "overly chatty" and thus you see a high number of your comments removed.
In any case, you shouldn't be making comments as a permanent record of anything on Stack Exchange sites. They are to be "used to ask for clarification on, suggest corrections to and provide meta-information about posts" -- not for providing witty remarks, "thank you" type posts or for long-drawn out back-and-forth discussions on questions and answers.