Why does E-ink displays have to do a full-screen refresh to remove ghosting?
The simplest answer is also the most obvious. When issuing new information for the screen, deciding whether each and every pixel needs to change takes longer than issuing a complete refresh, and twice the memory. Both frames would have to be available simultaneously for comparison. This could be done in hardware with a purpose built buffer, but it's still additional hardware. Since it is not capable of real time video, and is so power efficient anyway, we can skip the step of making that decision, pass in the new image inverted,  and then invert it again to make sure all the bistable elements were in the correct state. 

Edit: I should also point out, that the book file itself stores the sheet as an entire image - the whole image being displayed is different, and thus would require the special handling noted above.