Extant is quite different from existent.
It is derived from the Latin exstat, meaning literally it stands out. It is a term of scholarly. It related to documents and. writings that are no longer available even though they are known to have existed. So many of the volumes of the History of the Roman historian Livy are no longer extant (except in fragments quoted by others or in Reader’s Digest style ‘epitomes’.). The writings of the Greek Atomist, Democritus and other pre-socratic cosmologists are lost (apart from isolated citations) because they clashed with early Christian dogma. These are known works, such as a great work by The Alexandrian Theophrastus on the early Greek philosophers, which are no longer extant. But archaeologists keep discovering papyrus fragments, such as a complete comedy by Menander, found in an ancient landfill site outside ancient Oxyrrinchus in the desert towards the Nile delta. Others may be found. So they may be among the known works of literature, history, religion or whatever, so it is not quite right to say they do not exist. So we say they are no longer extant.