Physical books are the medium and the message. ebooks rely on an intermediary medium (ex: Kindle, tablet, or screen/viewing portal of some sort) which will shape content.
This becomes especially important if you consider The medium is the message and impacts how we perceive content.
A physical book can be any dimension it wants. An ebook fits another's dimensions.
In the case of Kindle and [[!DRM]] books, an additional intermediary is introduced in the form of the management [[!ecosystem]]. [[!Amazon]] makes it easy if you buy books from them but not others. A physical book is yours for good, the bookstore does not determine or suggest how you collect your highlights or where you record your collection. Ebook intermediaries such as Amazon do.