It’s a common misconception that distribution is required for infringement. Fanart that’s never shared still copies the likeness of a copyrighted character, and copying copyrighted stuff (outside a few very specific exceptions, e.g. fair use for parody, critique or education, making a single backup in case the copy you bought is damaged etc.) isn’t legal. As there are obviously no damages, no one ever gets in toruble for it, but that doesn’t mean it’s legal.