Indexers are a group of professionals who (for the moment) remain inured to the threat of computer programs replacing us. Certainly, there are computer programs that can help with the indexing project, but (to date) no program can achieve what the human mind can do.
The existing computer programs create a concordance of words in the text, listing each occurrence of a word. But such listing provides no context, has no gauge for the importance of that word on the page at that time. For example, in a book about World War II, a computer program might list every single occurrence of the word Germany. Imagine the list of index pages for that term alone!
A human indexer, on the other hand, would be able to subdivide mentions of Germany, into subheadings such as pre-WWI Germany, post-WWI Germany, history of Germany, pre-WWII Germany, arms build-up in Germany post-WWI, etc. These subheadings would provide some context for the hundreds of mentions of Germany in the text. Without subheadings and context, a reader would be condemned to visit each page to determine relevance to their search.
As Sam Leith says, “It would be a cliche to say that indexers are the unsung heroes of the publishing world. But unsung they generally are: no indexer usually expects or receives credit by name in books where everyone from the font designer to the snapper of the author photograph tends to get a solemn shout-out. And heroes they are, too: the index is, in any nonfiction book, more useful than almost anything else in the apparatus. It is a map of the text; a cunningly devised series of magical shortcuts that can in the good case save a scholar many hours of work.”
Human indexers can intuit and identify connections between topics, something that no indexing program has yet achieved. For that, you need a human brain.
Bite that, computer programs. Humans still rule.