I pray that you do not take this too offensively Masamune-1600 but Dream Zanarkand never was a location in the physical map of Spira. You might be referring the Zanarkand Ruins that's existent in the "real" Spira. But then again, Dream Zanarkand is just...what it says! A dream! That's the exact words of Bahamut's Fayth.

"This isn't a dream. That's because you ARE a dream. Your father, your mother, and the entire Zanarkand. They're all just dreams."

Well, they may not be the exact words of the Fayth but still they prove the exact point. He DID exist but only as a dream and not as a real person.

And it wasn't the summoning that linked Dream Zanarkand to Spira but the Fayth themselves. Since before the Eternal Calm, Sin reconquered on a decade basis hence the Fayth had to continue existing to give power to the Summoners. It was only when Sin has expired permanently that the Fayth may finally gain eternal rest and cease dreaming.

Since Zanarkand and its Summoners and Aeons lost the Machina War between Bevelle that took place a thousand years ago, the surviving citizens of Zanarkand transformed themselves into Fayths. And to somehow keep their Zanarkand alive, they gathered their memories and materialized them to create a whole new imaginary Zanarkand thus "Dream Zanarkand" came to be and Tidus and his family we're a fragment of that Dream Zanarkand.

"You know, we're pretty tired of dreaming. Do you mind if we rest?"

Although Tidus knew all along that Sin's permanent disappearance and Yuna's life would cost him his own existence, he still continued and sacrificed himself, not divulging to Yuna anything about his consequence. And now that Sin is gone for good, the Fayth may finally rest knowing that chaos and turmoil has finally been set aside. No longer do they have to dream about Zanarkand.