Because they’re protected by DRM that’s not currently possible. Or at least, not without breaking the law.
Hopefully at some point in the future changes to DRM will allow me to store all books side by side on my virtual book shelf.
But there are ways which this could happen while retaining the DRM publishers seem to love.
The first would be for Apple to add support for DRM’d books purchased elsewhere. I don’t see that happening.
The second would be for Apple to extend it’s iTunes Match service to ebooks, allowing users with legit ebooks to download the iBooks edition.
Would either Apple or the publishers be more likely to implement this?
Probably not, but if Apple could get permission for this it would give it a clear advantage over other reading apps which lock out books bought elsewhere.
Given its failure to date to replicate the success of iTunes, iBooks could do with some exciting, consumer friendly USPs to boost its desirability.