Three things to consider: packaging, price point, and pixellation.
Current laptops are basically ill-suited for book-type uses ---
they're too expensive and too fragile to be physically tossed around
the way a lot of books typically are, and their rigidity (and the way
the keyboard and pointing whatever-it-is physically relate to the
screen) make them really awkward to curl up with. Also, the
resolution on just about any currently available display screen is
dreadful by print standards, and it makes a real difference to the
readability of the text. So these are the problems that these guys
are working on.
Now, the evolution of technology might well take care of all of these
issues without ivory-tower impractical philosophers thinking great
thoughts about them. But it hasn't happened yet...