Why can we read a book but not a keyboard score?
Reading a book is totally different from reading a keyboard score.
Reading a keyboard score needs much more processing capability.
When we read a keyboard score we need to:
1. Need to know the notes.
2. Need to know how many counts.
3. Need to know the location of the note.
4. Our muscle need to be instructed where to press.
5. Need to read both LH and RH.
All of these have nothing to do with memorizing the notes.
I disagree with this to a certain extent. If we were talking about reading a book out loud, I would disagree almost entirely (based on the way it's worded), but as long as we're not reading aloud, the motor skills function is not necessary for reading. Once we start reading aloud, it is every bit as difficult as sight-reading piano music with one exception, which I will discuss at the bottom.
First, most people read around 250-300 words per minute. The average word has 3.5-4.5 letters in it. That's about 1200 letters per minute. Rachmaninoff's 3rd piano concerto, known for its many notes, still doesn't approach this figure. (Consider 30k notes, approx, at 43 minutes.. averages to nearly 700 notes per minute. Even if you take out the minutes the piano doesn't play, it averages less than 1000.)
We make up for reading by recognizing words instead of individual letters. However, for those who do not immerse themselves in musical scores, they do not pick up on groups of notes (chords, runs, arpeggios, groupings, etc). They simply see notes. Well, if you have to read all letters in order to understand words, you'd be reading books a LOT slower. So, now compound the idea that you don't recognize the "words" of music with the idea that there are nearly 3x the number of "letters", and the answer seems pretty clear.
Music is a language like any other, except we don't understand it as words, but rather process it as groups of sounds.
One major difference in processing power lies in the fact that we practice reading/speaking 8-12 hours a day since birth, but we certainly don't do that with music. Most of us read something every day we are alive, and I would wager nearly everybody speaks throughout the day. We also tend to speak in the language we read. These actions reinforce each other and make reading easier.
I would also wager that virtually nobody walks around reading scores of music for hours every single day. And I would quite easily wager that nobody puts their fingers on the keyboard as often as they speak. (For people with speech impairments, substitute sign language, writing with a notepad, etc.)
The second problem is voices. When we read/speak, there is only one voice we are trying to capture. It is either the voice of the character, your voice, or the voice of the person we're listening to. Processing multiple conversations simultaneously is something the brain is not hard-wired to be able to do. It can be practiced and learned, but you will still miss some things. The same goes for music: multiple "voices" create a particular sound with each passing bar of music, and the brain is not pre-wired to be able to deal with this. It has to be trained to internalize at least part of the process so it can consciously focus on one task.
All of this, of course, is disregarding the need to find the keys on the keyboard, which is both a similar, but also entirely separate, issue. Similar in that it is required for adequate sight-reading. Separate in that it has very little to do with the ability to literally read the "language" of music.