A genealogy going back to Adam and to God makes good sense from a symbolic reading of Genesis. If Adam stands for humanity, which is what the Hebrew word adam means, then ultimately one would expect every stream of human descent to flow into the sea called Adam.
It helps that Luke goes one link further: “the son of Adam, the son of God.” This last link makes it all but impossible to read the genealogy as simply conveying details about biological descent. God is not Adam’s biological father. Even metaphorically Jesus is the son of God in a different sense than Adam.
Finally, there’s Luke’s cryptic introduction to the genealogy: “He was the son (as was thought) of Joseph.” This suggests that what Luke is presenting is what was thought, not some divinely-revealed pedigree chart. Of course that statement may only relate to Joseph being the presumed biological father of Jesus, but I think it’s likely the rest of the chain after the first link is similarly what was supposed. If the first link is broken, who cares what the rest of the chain connects to? It wouldn’t make much sense to start from a presumed but biologically faulty starting point but then switch to divinely revealed information about biological descent.
I think Luke took this information that was commonly known and shaped it for his own purposes. He made all the names add up to 77 (with the addition of Cainan who isn’t found in the Hebrew text of Genesis) with key people and events coming at multiples of 7. Just like Matthew, he crafted the genealogy to suit his purposes. The dramatic differences between Luke and Matthew’s genealogies (both of which claim to show Joseph’s line) show how flexible this type of account can be.