Apple called them "folders" because they had this "desktop metaphor" where the files were pieces of paper and the background (what we call the "desktop" today) was like a physical desk.

This is also the reason GUI programs typically have a white background. It's like you're typing on a typewriter and the background is made of paper.

Prior to GUIs, it was common to see black backgrounds and glowing text, because that was easier for the hardware than the opposite.