Claverhouse said:
I dunno; if every site insisted the art had been first posted somewhere else I foresee a dreary paradox.
That's like saying that sites Pixiv and DeviantArt are pointless as danbooru and Sankaku exist. They're image hosting sites specifically set up for uploading user-created art -they have art management and gallery features places like Imgur and Danbooru don't have, to be blunt.
We're not really set up for hosting art a user creates - once it's uploaded they have no control over it or how it's displayed. Pixiv and DeviantArt let the Uploader control everything about the image - removing it, revision, restricting access or resolutions displayed etc.
Plus, Danbooru was originally set up as a repository of moderated (high quality) anime style art, not as an artist's upload site. We don't give the artists any control of the content other than to remove it completely - which they have to request. On an artist hosting service, they'd have all the access controls they wanted without going through anyone else.
Every site has their own rules - a warning against self uploading is specific to Danbooru.