Because the Bible isn’t a one-shot document that God typed up in MS Office in an afternoon. It’s a compilation of numerous texts given to different people groups, through 40+ human authors, over a 4,000 year period. That means each individual book can’t be ready as a simple list of “do this, don’t do that,” but rather must be interpreted in its own literary, historical, and cultural context.
That is, Song of Solomon is Hebrew Poetry, written at the height of the Davidic Kingdom to a Jewish audience. You can’t read it with the same mindset as 1 Corinthians, written by Paul to 1st century Gentiles as an instructional letter.