Why do we have a tag for unknown_artist? If the artist is not tagged then it says "none", which is obviously not true, it simply means that the artist is unknown. I guess it's a lot easier to search for, but then, couldn't you just have unknown artist tagged as default? I just don't see the difference between these two situations.

From the tag description: "Either the artist signed with a squiggle, or there was no name on it, the uploader did not provide the source, and/or used an unclear tag and did not set it as an artist tag. Remove when the image gets tagged with the artist's name or nick. Source link is only to assist finding the original source image, not to ascertain the artist. Commonly the poster is unsure or just lazy.

Basically it's so added in the hope of someone knowing the artist/source, and adding it.

While you can search for arttags:0, most users aren't entirely familiar with the metatags and so the unknown artist tag helps people find images that need the artist found.

And I guess it would be a lot of work for little benefit to add it as default.