Answer:
A choice.
Explanation:
You may notice some people, movies, etc using green screen on behind-the-scene.
This is because once the background is green or in one color, the person who edits photo or video can clear out the green background and makes the video really realistic.
So what happens if we don't use one color background? Well, it'd be super hard to edit the background and the video might come out weird and messy.
The answer is spirit of antiquity.
