Because when the rules for electronic delivery were being written in congress and later legislated in the courts the publishers make damn sure to close the "library loophole" and that pesky "doctrine of first sale". The public didn't really have a seat at the table so the laws were written with a heavy deference towards the interests of the publishers. There wasn't a partisan divide either, lawmakers came together to perfect harmony to allow publishers to bend the public over and take them without lube. If you are a congressman the last people you want to anger are the ones who own the newspapers and TV stations.