It's the same set of reasons I buy books from Apress, Pragprog, NoStarch, Packtpub and Manning (to name a few others).
I have tried Safari Online through an employer subscription, but I just cannot get used to reading long books on a browser and hence don't use it (don't even talk about apps on other platforms). I need my books in specific formats that I can use in book reading apps across different platforms. I don't like being connected all the time just to read a book. For me reading books is mostly an offline-like activity (except if I have to search about something online).
Books are also very different from other kinds of media that are more of an ephemeral nature, like music, movies, TV shows, etc. Though many technical books might become outdated quickly, there are many books that do contain almost timeless pieces of information and wisdom. I don't buy books on Amazon. I don't buy books in Apple's iBooks store. There is no way to know upfront if something has DRM or not. I don't like supporting DRMd content for books.
O'Reilly, if you're reading this, your decision to stop selling eBooks like you used to is a very poor one for your readers. You may end up making more money from unused Safari subscriptions, but you won't have the fan following you had before.
As for comparisons with Pluralsight, I despise such models that are filled with DRM and restrictions.
To Apress, Pragprog, NoStarch, Packtpub, Manning and others - please do not change your models and remove DRM free eBooks in multiple formats from your stores. I, for one, will stop buying whenever something like that happens. Subscription models are very poor in user experience and don't provide adequate freedom to read whenever/wherever.