plural names of cities

Is it known why the names of many prominent cities in Greek and Latin are plural nouns? e.g. Ἀθῆναι, Δελφοί, Θῆβαι, Cannae. Under the heading for Ἀθῆναι, my Lewis & Scott says:

used in pl., because it consisted of several parts

That seems really unlikely to me. Is there a consensus on this?

This doesn’t solve anything, but here is what Smyth (1005) says:

“Names of towns and parts of the body are sometimes plural: Ἀθῆναι, Athens, Θῆβαι, Thebes, στήθη and στέ?να, breast (chiefly poetic). The name of the inhabitants is often used for the name of a city: Δελφοί D.5.25.”

Right after this he talks about the plural being used to lend dignity. The LSJ says the same thing about being composed of parts.

Well that’s what I was taught, at least for Athens (can’t remember about other cities). Many little settlements that became one big city.