Not easy as you would make it out to be. Can you imagine the manpower required to have someone TYPE each note, (they would have to be verbatim), into a computer system. The officer WOULD need to access each notebook each time he is required in court, (MUST have the original document). Then you have someone "trying" to decipher the writing of another, (I have reviewed notebooks as a supervisor that bordered on being illegible..lol). Yes of the officer can't produce notes, (and the lawyer or accused requests to see it the case gets tossed. Then you have to build and monitor a "secure" facility to house all the books, Lawyers would have a hay day, questioning the voracity of the secure facility.
Most officers keep their notebooks secure, and it is RARE for one to be stolen, (it really has no value to anyone other than the officer).