A family of Microsoft word processing software products for creating web, email, and print documents.
It's not an explanation. It's not even a very good excuse. Quark and FrameMaker have been around longer than "online documents" as well. So has PageMaker (the precursor to InDesign.) These programs are designed primarily as a replacement for old hand type-setting when printing documents. Desktop publishing software meant for "print publishing." ("Publishing online" is a more recent usage, initially promulgated primarily by bad Microsoft HTML WYSIWYG programs.) This hasn't affected other software development teams from keeping with the times. Even Wordperfect has a means of hyperlinking index entries. The only reasons that MS would develop code to create hyperlinks on a generated table of contents and NOT develop code to create hyperlinks when generating an index would be oversight, laziness, or some combination thereof.
I understand you have no influence on the development team. But you shouldn't try to make excuses for them either.