Cause they are abstract.
Updated
Topic: implication: white_background, coloured_background, black_blackground, background_gradient, background_pattern, texture_background to abstract_background
Posted under Tag Alias and Implication Suggestions
Hrm. "(Fine Arts & Visual Arts / Art Terms) denoting art characterized by geometric, formalized, or otherwise nonrepresentational qualities"
background and texture i could see fitting in, but monocolored backgrounds or simple backgrounds wouldn't fit into being abstract so much.
Updated by anonymous
123easy said:
Hrm. "(Fine Arts & Visual Arts / Art Terms) denoting art characterized by geometric, formalized, or otherwise nonrepresentational qualities"background and texture i could see fitting in, but monocolored backgrounds or simple backgrounds wouldn't fit into being abstract so much.
White/coloured/black/gradient aren't used to describe monocoloured backgrounds regardless. (white/black/coloured is used to describe the flat colour backgrounds, and simple background is a little vague, requiring a precise definition before I can comment on that). Regardless, it would be better if we talk about this in http://e621.net/forum/show/18338
Updated by anonymous