Almost everything an issuing authority does rests on a photograph and a set of details that the applicant provided. An examiner works from what was sent in, usually without ever meeting the person, and at a volume that leaves very little time for any single file.
Both halves of that are now straightforward to manufacture. A facial photograph can be generated or quietly altered so that it survives a visual check while belonging to nobody at all, and an identity can be assembled from genuine details taken from several different people, so every individual detail verifies correctly while the person described has never existed.
The consequence outlasts the decision by years. A passport or a visa issued on a false basis becomes a genuine document, accepted everywhere, and used to obtain everything else. The checks below examine what was submitted and give an examiner the findings behind the result at the only point where refusing is still an option.