Most people aren't active readers. Goodreads users are active readers. Amazon bought Goodreads (and other sites like Shelfari) because that's where the readers are.

I read 74 books so far this year. Most of them were purchased from Amazon.

Also, Goodreads has an active review community, and good "by reader" data for recommendations (as opposed to Amazon's "by account" data. If Amazon were smart, they'd be working very hard on improving recommendations with their Goodreads data.