1. Convenience. Most of the brethren probably don't even consider other holy writings as being holy. We have lodges in Atlanta where the Koran sits next to the Bible. GA code requires the Bible, but permits others as well. CA code requires lodges to have six (if memory serves) different scriptures available in case a brother requests one of them. Whichever volume is used, it is used to
represent man's idea of God, even if that particular volume isn't the one each brother uses.
2. I prefer the explanation presented at
http://freemasonry.bcy.ca/texts/theletterG.html
3. Without a belief in Diety and a concept of spiritual accountability, of what use are the obligations?