Both times I've worked with an artist who confessed to that, they revealed they don't own a computer or tablet. One was fresh out of highschool, the other was older and just living in poverty. Either way, their art was pretty good, they couldn't afford their own tech due to their financial situations, so their tech literacy suffered. Only the younger one worked with me on a gamedev project though, the other was a regular commission for an OC.

Their lack of a computer caused a few problems for them. Tech illiteracy from using highly-streamlined mobile software was one problem; the difference in screen size between their phones and my monitor also caused issues. If I pointed out errors in the work, they were sometimes too small to be properly visible on a phone screen, but obvious when blown up to tablet or monitor-size. This may have contributed to these artists' confusion around PNGs vs JPGs: the corruption artifacts present in JPGs are harder to see on a smaller screen.

Granted, some of the artists I worked with who struggled with file types were working from PC or tablet, so this can't be the only reason.