I'm not sure if you know the difference, but doujinshi are independently published, whereas tankoubon (books) are commercially released manga. Books are almost always made up of chapters that were previously released in one of the publisher's magazines. So for instance,
Ouchikaeru's works appear in Comic Kairakuten, which is published by Wanimagazine. Once there's enough content, Wanimagazine compiles those chapters into a book.
By contrast, it's rare for independently released doujinshi to get compiled into a book by a commercial publisher. One of the few exceptions on here I can think of is
Futabu! Mix, which was published by Akaneshinsha despite mostly consisting of doujinshi releases. It's also possible for the individual creator to compile their works into a bigger collection, but those are often much more costly compared to commercial manga.
As for your specific question of why the
TSF Story Append volumes aren't part of the book; I'm not personally too familiar with the series, but given the name "Append," I imagine they were released by the artist
after the the book. We mimic the Japanese release, so if something wasn't there in the original book, we won't add it.
TSF Monogatari is already a massive tome on its own (240 pages when the standard is around 210), so there's no way we could've slapped on another 345 pages with the Append series. That combined is enough content to fill three volumes.
Finally, we (FAKKU) do publish
box sets where we compile doujinshi releases. This is always a possibility for any of the artists we work with, but it's far from a guarantee. The series in question,
Hypnotic Sexual Counseling, is not published by us, so whether or not that gets a physical release is completely up to
Irodori Comics. As far as I know, they have yet to publish a physical compilation of any of their releases.