Because it makes no sense at all to retain information read in a book when forgetting the same information minutes later that was read in what is essentially a thinner book.
I learned a whole bunch of stuff on Physics at school, stuff about Vectors and Scalars, etc.
There has only been one point in the past 5 years i've been able to remember a word of it - and that is after a little prompting on the forums - just a question that was asked and a few people half remembering bits but not enough to come to the answer to the question presented, the bits that everyone half remembered was enough to trigger enough of a memory to answer the question.
The fact is, the human brain doesnt have "Always on" access to all the information we have in our head. Sometimes we need a little prompting to get access to the "back room" of our memory. This is what magazines do, they stimulate the mind enough to help shift those memories into the front of the mind.
As this makes me think about it, its slowly coming back, but not enough yet that I can remember the question in full, but in the last few minutes I remember it had something to do with 2 forces and using Pythagurus theroem to work out the net effect.