Actually, no - they are named individuals in historical records of great antiquity. Whether those historical records are reliable is a question for historians, given lack of contemporary corroborative testimony of their existence (but not of their historical verisimilitude, which for Abraham and Moses has become more solid the more archaeology has discovered). But the biblical texts are evidence, and evidence of just as much historical weight as any other ancient text.
But the problem of single sources affects all ancient history - a majority of the characters in ancient histories are not corroborated by any other sources. A majority of ANE kings mentioned in non-Biblical sources have no archaeological corroboration. Ergo, until further evidence refutes them, sources are generally cautiously assumed to be reliable by historians, or there wouldn’t be any history of those times.
For example, nearly everything we know about the existence of the father of written history as we know it, Herodotus, comes from a 10th century CE Byzantine source - 14 centuries after his death. We do, of course, have his works (also from a 10th century manuscript, and from only 15 small fragments and fragments of quotations back from the 1st-4th centuries CE).
But of course we have writings attributed to Moses, too - 103 manuscript fragments from the Dead Sea Scrolls alone, which of course date from roughly the same time after his death as the main manuscript of Herodotus after his. If Moses is fictional until proved otherwise, so is Herodotus.
And a mere bit part player in Moses’s story - Balaam son of Balak in the Book of Numbers - has turned up as a genuine non-Israelite prophet in an inscription at his cult centre, in a Canaanite dialect.