Magical information actively dislikes being duplicated.

If you memorize a spell off a scroll, it disappears from the scroll. If you transcribe that spell into your spellbook, it disappears from your mind. If you cast a spell -- the act of effectively copying it onto the world -- it disappears from wherever you cast it from, be it your mind, a scroll, or a charged item (where it's held in compact form).

This is why spellbooks have to be written in a special language, using special inks made from the blood of dragons, amongst other things. It took a long time for wizards to find a way to record spells so that they don't disappear when they're copied or translated. The method they hit on isn't cheap or easy, but it is reliable.

A Blessed Book is enchanted to allow spells written in ordinary ink not to disappear.