Fair points. I am certainly not a Photoshop expert. You are probably right about the extent of scanned originals.
Labels like Prestige, Jazzland and many other small to mid level labels of the day did a terrible job of keeping original cover materials, if they ever had them - many times it went straight to the printer and no one cared to get it back. Verve and Impulse stuff probably went up in smoke, as Atlantic probably did.
Major labels tend to have stuff. I understand that WEA and Sony owned stuff is mostly all there. Blue Note archives contain pretty much all the original imagery, but reissue labels must specifically ask and force the label to dig, as default is to send a .jpeg of whatever quality is on file. I would imagine in many cases it is just easier to work on what is given, particularly if running up against a release date, rather than wait for the label to find better.
Given how many times record labels have changed hands over the decades, nothing is a surprise. And let's be realistic - 50, 60 year old images and negatives are not always in great shape.