The reason why background in traditional animation is so beautiful is because it is made by a person sitting in front of a sheet with paint. It isn't a subjective thing, when you can't zoom in (like in digital), then you have an overview about what is gonna be visible and how much visible. So you will end up being very concious about how to design, about where to put details and where not and everything is gated by the size of the tools and the size of the sheet.
Another important aspect is how much you can work on a background, you can't take shortcuts so you wouldn't waste time painting something that won't be visible.
When cartoons used to feel like dreams you could walk into.