Those programs can store the diagrams in sgf format, which could also be used to create a kind of interactive electronic book… For about the same effort currently required to create static diagrams for paper books one could create dynamic diagrams which would be easier to understand.
No, the point is is that it is not the same effort. People who have tried so far have found that the amount of work required for e-books is much, much higher. I agree that on paper (as it were) it shouldn't be much different, but the reality is that it is very different, which means there is a lot of work in redesigning and rewriting. This has largely to do with the way variations and current readers work in sgf, and the fact that conventionally written text or pointers to variations disappear at crucial moments. There are also difficulties such as being able easily to compare one diagram to another int he same view. The upshot is that text for e-books really has to be written ex novo, and that is just not worth while at present.
Of course a current paper book can be converted to a pdf file if you count that as an e-book, but that is tantamount to giving it away free. The pirates affect the go market much more than most people realise. It is one of the most important reasons books are not reprinted, for example. But actually the quite legal business of re-selling or lending go books is also a deterrent. People sometimes complain about the high price of go books, but again it is a special niche market and you have a high chance of recouping some money by re-selling. But that also works against re-printing.
The implication behind a question like where you can get an out-of-print book for less than $100 is that you have a right to such a deal. You don't. And remember also that the author gets no benefit from the second-hand price. There is one minor book of mine where a re-seller gloatingly made more on his one copy than I made on the entire (small) print run. If you want a steady stream of good books that stay in print at reasonable prices, you need to ask whether enough is being done to help the producers. The biggest single way to help is easy and free. There are virtually no advertising outlets, so actively talking more about specific books (as in this thread) would be a major boost.