Get Info does not allow me to manually add album artwork

I assume that there are legal restrictions on the way in which iTunes can get artwork automatically from any source apart from the iTunes Store (which is what the right-click > Get Album Artwork function uses). For example, even though the Gracenote service that iTunes uses to match CDs and retrieve metadata (artist, album, track titles, etc.) also has artwork functions, iTunes does not make use of these.

For manual addition of artwork, there are three cases where the image you select may not be correctly embedded in your media files:

• your media are in a format that doesn't accommodate embedded artwork - typically WAV files. To embed artwork you'll need to convert to another format - Apple Lossless or AIFF if you want to preserve lossless quality, AAC or MP3 otherwise.
• your media files are read only - to fix this, use Windows Explorer to find the folder that contains your files, right-click and select Properties. On the General tab there's a check box labeled "Read-only" - if this is checked, or is grey (sometimes blue), click the box so that the flag is unchecked and the box is white. Click OK, and then OK again when the "Apply changes to this folder, subfolders and files" option selected. Now try adding the artwork again.
• Windows permissions issues are preventing iTunes from updating your media files (there's some anecdotal evidence of a change in this behavior in iTunes 12). See turingtest2's notes on Repair security permissions for iTunes for Windows for advice on fixing this, then try adding the artwork again.

That doesn't change the fact that Apple may be working with the constraints of agreements that prevent it from offering automated artwork discovery and import except through the iTunes Store. Otherwise there's no logical explanation for the fact that iTunes doesn't also make use of the artwork image functions of Gracenote, from which it is already retrieving metadata. Also, the vast majority of album covers are not in the public domain - even Wikipedia has to invoke "fair use" clauses to include album covers in its pages.

"Apple needs to ... allow users to attach album artwork to iTunes files no matter what audio format they are saved in"

They can't ... the WAV format has absolutely no provision to include artwork, or any other metadata. WAV files are simply a container that allows PCs to "see" the data stream on an audio CD as "files". There's really no need to use WAV files in iTunes - Apple Lossless would be a much better option as you get exactly the same audio quality (it uses the m4a container for standard "red book" PCM data, using lossless compression to reduce file size) and allows artwork and metadata to be embedded in the media files.