Why don't ebooks have indexes?

Must admit, this is a sore spot for me - I create book indexes part time as a freelancer.

I'm one of those people who enter a non-fiction book from the back - from the index! If I know what I want, and I can find it in the index, I will dive into the book... if it's not in the index I can save myself a lot of time by just finding another book. But if the index is missing - or in a lot of ebook cases, it isn't linked to the text... page numbers just won't work for an ebook - using search just makes the effort tedious.

I'm currently using Adobe InDesign to help an author publish his book, and he wants to go ebook, rather than print, to start with. For print publication InDesign does have a rudimentary index generation function - nothing like my professional indexing software, Cindex, but useful when I've already written the index in Cindex, and can just input it into InDesign. But for an ebook... I've been Googling for how to translate the index into a functioning - click an entry and go into the text - index. If I can figure it out, it doesn't look like it will be easy.

So it looks like the available software to create ebooks is not up to the task of creating an index. I think this is a big gap that should be filled. I hope more people will realize this and start asking for indexes on ebooks.