It's because people keep taking screenshots of the image and sharing the screenshot instead of the original image file. It's like making a copy of a copy of a copy until it looks like garbage.
Stop right there criminal scum, you are not allow to publish original copyrighted works, you are stealing from the artist's mouth by squandering his market value !
because they get downloaded from say reddit and then reuploaded again a year later or so which since most sites/services compress files uploaded they get worse and worse quality