Because if what comes out of the speakers is the song you are supposed to be playing it makes no difference if you know it is in A minor switching to Bsus9 no 3 for the bridge, and it is a pattern of descending 4ths. If I play something from a record without looking at notation of any form, do I know what I'm doing? Do I know that the 3rd fret on the E string followed by the 6th fret might mean that I've just played a minor 3rd of G or does it really matter because the listener doesn't care.
for what its worth I think knowing theory and reading are not related. Knowing the fretboard, knowing the triads, scales, modes etc doesn't need you to be able to read dots. I can play a piece of tab and know the relationship of the notes just as you can from a piece dot based notation. What is missing is the rhythm part and for most pop music the addition of the record to the notation will solve that.