Maybe it's because they don't know what kind of FG you'll actually get. It depends too much on the brewer.
You could get a decent estimate by assuming 70-75% attenuation, then take OG minus FG times 1.3, roughly.
NB's Nut Brown has an OG of 1.044. You get down to ~1.012, 32 x 1.3 = 4.2% AV, more or less.
ADD: very roughly, the OG gives you the ABV. A 1.060 beer is around 6%, etc.