Not sure this has been covered, but perfect pitch combinations and harmony is possible, because two or more frequencies at the optimal comma intervals, can reinforce each other, and lock into perfection, for just intonation or other colors* of intonation. Guitarists do this by pulling one or more strings into pitch so two or more reinforce each other. Violinists and others do this as well. Better brass and woodwind players do this by compensating, as do better vocalists.
Open string tuning is a compromise, but guitarists can and do push/pull the neck and strings to lock into perfect fifths, thirds, sixths, etc. And its the reason there are no frets on unfretted instruments. Its easier to create the proper pitch with no frets. For instance, there are several "B" notes on a violin, in the same 1st position 1st finger second string. There's the B that is a perfect 3rd with a G note, there's another that is a perfect fifth with the E note, there's another that's a perfect fourth with an F#, etc. The finger placement is very close but different for each of these. You can find all of these on a guitar too, pulling and pushing the B as needed till you lock in.
The book I recommend, written by a PhD but incredibly well written for the non-mathematician, is How Equal Temperment Ruined Harmony by Ross Duffin. Excellent book gift.