It's not a modernization question, it's a political battle. See
http://www.nytimes.com/2013/05/01/opinion/e-books-libraries-...
for example. Some points from there:
1. It's only been in the last couple years that all of the big publishers would actually sell eBooks to libraries.
2. It's not selling, it's licensing, with severe restrictions.
3. They charge higher rates for the eBooks to Libraries than they do to the general public.
4. Some publishers only offer eBooks to libraries when the books have been out for six months.
Not in the article, but also important: publishers claim that libraries cannot simply digitize and loan out e-versions of their existing book catalogs--they claim that the eBooks are a different product, and that libraries have no license to do that. That would probably not stand up in court, but libraries aren't eager to antagonize the publishers, from whom they are still trying to get regular eBooks at regular prices.
The article is kind of ill-informed on this, too, I'm afraid. The difference between filesharing and public libraries is that public libraries operate within de facto and legal restrictions that don't apply to filesharing. A much better comparison, that fits the author's claim that the only difference is efficiency, is home taping and CD-copying.