Glenn Horowitz built a fortune selling the archives of writers such as Vladimir Nabokov and Alice Walker.
Different collectors buy different things- some like books owned by specific people, others want works (drafts, letter, editions, etc) of an author.
Pristine is for some, a book that's been lived in is worth more to others who look for margin notes.
Of course, this has the advantage of keeping prices down for those of us looking for those other editions and conditions, but it can at times be rather perplexing. I was once in an auction where it was evident that the auctioneer was also surprised by a case of this: in an auction with a first edition by an American trade press, and a first UK edition from a year later by a particularly notable private press, with a smaller print run, typeset and printed by hand by notable historical figures and friends of the author, and an estimate of around four times the first trade edition... the UK edition sold within estimate, and while the first edition sold for significantly more than the UK edition, vastly over its estimate.
In another case, I had a bit of trouble finding sellers online who even noted the edition of a particular 19th century book in its description if it was not a first edition, despite the second edition being at the center of a significant historical legal drama, being nominally banned and ordered destroyed, and making a mess of British blasphemy law in a case where no one, including both the government and the prosecution, wanted the publisher to be found guilty.
My guess is that many book collectors will set a particular goal, for example, collecting first editions or author-signed copies of a particular genre, set of authors, etc, and will follow that goal, rather than acquiring individual books for individual reasons.
More generally, collecting communities often seem to fall into purely seeking rarity, placing the highest demand on the items with the least supply regardless of why the supply is small. Thus at an extreme in book collecting, for example, you have collectors who see entirely uncut pages as being preferable, despite it making the book unreadable. I have a friend who is fond of antique pens who expressed disappointment that in seeking the rarest pens, the community often ended up placing the highest value on the worst: the pens with bad designs that didn't work well, the variants and colors that were particularly ugly, all the models that sold very poorly and were quickly discontinued, and are thus rarer.