To make this distinction kind of suggests a misunderstanding about the nature of translation.
Well, you have to take into consideration the significance of the text in many people’s lives, especially those who were doing the translating. Religious matters are often judged by different criteria than secular ones.
In addition, the Bible contains a number of warnings against any attempt to add to (or subtract from) it:
Deut. 4:2 Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the LORD your God.
Prov. 30:5-6 Every word of God is pure: he is a shield unto them that put their trust in him. Add thou not unto his words, lest he reprove thee, and thou be found a liar.
Rev. 22:18-19 18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
Given these injunctions and threats, it’s not too surprising that the translators took pains to distinguish words that were not directly attributable to the texts they were translating.
I recall, but have not searched for, a previous discussion of this non-intuitive use of italics, and somebody mentioning a former pastor or schoolmaster who, when reading aloud, assumed the more usual convention that the italicized words were to receive extra emphasis. I suspect this is actually a rather common misapprehension.