Mythological Interpretation

Certain myths have the obvious purpose of explaining why a place or thing has the name it does. The Hellespont, for example. However!!?!?!! If that's the case, why do so many names not have myths appended? Do we just not have them? Were the sources of most names widely known or obvious similar to the approximately 5,000 places called Newcastle after translation? The likeliest cause is that the Hellespont was actually named after or at least by somebody a persuasive mythographer didn't like and wanted to erase. The second-likeliest is that mankind came here from another world and carried over most names from there but needed a few new ones. Third-likeliest is that all such myths, when put together and interpreted, tell us where the treasure is, and that's the one on which I'm banking.