Tempered tuning indeed divides the octave into 12 half-steps, but a huge amount of music uses only 7 or fewer of them for long stretches (or entire pieces) with a few exceptions. So think of the lines & spaces as being a compressed representation that doesn't waste vertical space for the tones that a piece isn't going to use.

Me, I love standard notation. Common chord voicings and interval patterns stand out as easily recognizable patterns on the page.