Every damn one of them has some inane lock-down that any student you ask will run up against, but has no conceivable purpose. And then there's the TOC at the beginning - less than 1/4 of the ones I've encountered even link to the pages they reference, and all non-linking ones I've encountered are incorrect because PDF pages are != book pages (they include the cover). Far far less likely for the index or glossary to be linked (though then there's searching).
They're not selling because publishers are doing squat to make them worth buying. Or even existing beyond being a faster way to search.