Because you don't know what you don't know, and neither does Buy'n'Large. In a second hand bookstore you'll come across things in the same category that are useful and important but don't come up in a recommendation engine because they're not popular. The deeper into a field you go, the more noticeable this is.

Also, the second hand bookstore isn't maintaining a secret police file on every micro-action and momentary impulse you exhibit while inside their property.