“Art” is something philosophers have spent centuries trying to define, sadly with no satisfactory result (a debate that is far beyond the scope of this article). But illustration, while it covers a broad range of image-making, does have very distinct meanings, and it is very different from just illustration.
To reiterate: as an illustrator, your job is not to demonstrate your painting techniques, color mixing, or your mastery of the male nude — only to find the most efficient, clear, and direct way to make the viewer feel and know what you want them to feel and know. Do nothing extra, because extra stuff (be it extra details, people, trees, colors, lines, etc.) in your illustration could only hamper this process. Just like writing an overly wordy essay or an instruction manual with too many details makes information difficult to digest.