On Project Gutenberg: the children may prefer more modern books that reflect their life, interests, and values. Older books also tend to be text heavy and a lot of Project Gutenberg titles are text only, especially the ones transcribed early on. This does not necessarily reflect the needs of the modern child, particularly younger children and reluctant readers. (Yes, they call them that.)

On Libraries: libraries do not always have the money to purchase the books that their patrons are interested in. Consider how often you've searched for a book, only to discover that your library does not have it. Or consider how often you've found a book at your library, only to be placed on a waiting list.