Regardless of the key a book or person chooses to present a song in, each singer doing the song still has to make the key choice on their own. Keys given in songbooks are arbitrary, and keys picked by professional singers are according to how their own singing range works with the requirements of the range of the song.
My own Bluegrass Songbook's, and others’ emphasis on the key of G reflects the then-reality that most bluegrass singers were male, and most male voices are comfortable with most bluegrass songs in G, or perhaps slightly higher keys like A or B, which can be played easily on guitar and banjo with only a capo and the knowledge of how to play in G.
In reality, every singer picks the key for each song that best suits him/her (with some allowance for the versatility in keys that they and their fellow musicians can handle). In bluegrass, as in most music forms, the same singer might sing in a variety of keys, depending on the range of each song. At some point every musician needs to learn to *transpose*, to put each song in its optimum key.
Assuming a knowledge of transposing, in my songbook, I could have just given the chord number values, as in 1, 4 and 5 chords (I'm guessing you may have heard of that terminology for chord changes -- also covered in my Songbook), or I could have picked a key, with transposing left to the individual singer. I mostly picked G as a default, based on the knowledge of what usually works best for the typical male voice and what's actually the most common key in bluegrass.
There's a lot more on this subject in Bluegrass Songbook, in two of the appendices, especially Singing And Playing In The Right Key For Your Voice. It discusses the historic precedent for the common use in bluegrass of the key of G and its "capo-able" variants such as A and B.