The index is the last part of a non-fiction book to be created, assembled in a narrow window shortly before the book goes to print. The timing is forced by what an index actually is.
An index depends on final page numbers
Every entry points to a page, so the indexer cannot begin until the text has been typeset and the pagination has stopped moving.
Any correction that adds or removes a line can push text onto a different page and invalidate entries that were already correct.
The work therefore waits until the proofs are locked, which is one of the last stages before manufacturing begins.
The window is short by construction
Printing slots are booked in advance and publication dates are already announced to retailers, so the schedule cannot absorb delay at this point.
Indexers routinely work on a few hundred pages within a week or two, which is why the job is usually done by specialists rather than by the author.
Authors who insist on indexing their own books discover that the task arrives exactly when they are least able to give it weeks of attention.
Indexing is interpretation, not extraction
A useful index records concepts, not words. A discussion of migration may never use the term, and a passing mention of it may not deserve an entry.
The indexer decides what a reader will look for, chooses the vocabulary those readers will use, and builds cross-references between related ideas.
This is why an automatically generated word list is a poor substitute, and why a good index is recognisably a piece of editorial work.
The cost often falls on the author
Many contracts make the index the author's responsibility, either to compile or to pay for, with the fee deducted from royalties.
That arrangement gives authors a direct incentive to skip it, particularly on books with modest expected earnings.
Academic and reference publishing treats an index as essential, so it is budgeted for, while trade non-fiction treats it as an optional extra.
The absence of an index in a serious book is therefore usually a cost decision rather than an editorial judgement about whether readers need one.
Digital editions did not solve the problem
Reflowable ebooks have no fixed pages, so an index has to link to positions in the text rather than to page numbers.
Search is a partial replacement but a poor one, because it finds words while an index finds subjects, and readers looking for a subject rarely know which word the author chose.