air has moisture, you do not want to create a sealed chamber where condensation may occur due to temperature/pressure changes
the properties of paper are also relevant I believe but beyond my pay grade here
Bird is spot on. You do not want it to be air tight because of this, as well as the paper will give off gases as it ages. This is a standard part of time, it is essentially decomposing. While the slabbing slows it, it will not completely halt the process. If you trapped the gases the paper is releasing in with the book it would speed up the decomposition, if I am off on this and someone has a better reason feel free to correct me (but this is my understanding). So instead the slab allows for the gases to be released.