Oddly enough, the majority of the "texture artists" in any modding venue utilize the textures that were included originally in the game itself. For example, the initial "HD" texture packs were the base textures batched processed in <enterimageeditorhere>, increasing the image size, then applying several filters to de-pixelate,sharpen and otherwise alter the original.. Original (read: derivative) artwork, my arse... Golden rule to texture creation and editing is you start big, then shrinkdown and compress the image to avoid file bloat.
In terms of meshes, any actual new and done by hand, then imported into the game itself constitutes an original piece of work and can thereby be claimed as intellectual property of the author. If even there is anything reused within the end product, that author has no right to call it their own. Taking a mesh, adding for detail or deleting faces/polygons to make it "skimpy", and then uploading the revamped item and finally calling it theirs is stealing in it's own right.
Upon releasing the CK, did Bethesda at any time say "No, you can not change anything in the game" or "No, you cannot add anything to this game"?. Did they take any steps to shut down Nexus or any other site that catered to modding the hell out of their game, even before they did release it, to protect their intellectual property? No... they fully intended the game to be modded by any and all that showed up to the party.
TL;DR version :
If Bethesda didn't give a poo, why the hell do these modders think they should (Short of the ones that actually did model, texture and/or script their own content?). There's this thing called creative/artistic license that suggests people should give credit where credit is due. The compilation author did just that.