“Adults in the 21st century are photographically literate enough to ‘read’ any photo as long as it is not doctored and not misrepresentative.”

Why do viewers ask whether a photo is “real”?

Because if an image is not what the viewer considers a “real” photo — regardless of how the viewer defines that term — then the viewer will not have faith in their usual mental tools for “reading” photographs (see italics above)—

— and the viewer will not be confident that they can “read” the photo that they’re asking about, so they will trust the photograph less.