Comments are not stored inside the book -- they are stored in data files related to each book (for Kindle, look for .mbp1/.azw3r/.azw3f files, which may be in a {book}.sdr subfolder) which are I believe binary files anyway.
I believe Kindle for PC stores annotations data also in a file called "My Clippings.txt" which is essentially a log of annotations taken (and only gets added to on the device that took the notes). I know this file exists on e-ink Kindles.
Try looking at the Annotations plugin for calibre, to import them into a custom column. Or
http://clippingsconverter.com to turn into various formats.