Because it offers the content to everyone anywhere for free without authentication or a limit on the number of concurrent copies available.

Is this a good tradeoff between protecting IP to incentivize creation through monetization and the various societal goods of making it widely available? I don't know, but it is certainly a different point on the continuum than traditional libraries.

> Is this a good tradeoff between protecting IP to incentivize creation through monetization and the various societal goods of making it widely available? I don't know

The actual current status-quo is. Despite the most of the worthy books being available on pirate websites, people still buy books. Publishers still are not bankrupt. Even independent individuals post "I wrote a book" here every now and then, link to DRM-free purchase pages and seem happy.

I personally spent many hundreds dollars buying DRM-free ebooks on GumRoad and HumbleBundle (despite most of them being on LibGen!). I also bought numerous hardcover paper books after reading their pirated ebook versions.