The problem is that concept_art is totally indistinguishable from any other kind of art. I can draw a pretty picture and call it concept art for some new universe of mine, and that might be technically true, but it's not necessarily good to have that post showing up in search results. I presume most people who use the tag use it in searches like zootopia concept_art or video_games concept_art.

Should we introduce a notability requirement? Tag concept_art only for official concept art of notable works?

What if my post also contained a detailed textual description of the concept? What if that text is instead in the post description? Does that change anything?