Sin and Zanarkand SPOILERS

[Tavrobel]

Exploration outward of Zanarkand wouldn't be affected by that, because they don't know about Sin. The only possible explanation is that they don't care what else is out there, but that still feels weak. And why would Jecht go, if they were all designed to have minimal desire for exploration?

Sin was present starting immediately before the end of the war. Sin ended it. It was known.

"There is a legend, you know."
"Just before the horrible Sin appeared..."
"a terrible war raged between Bevelle and Zanarkand."
"When the armies of Bevelle attacked Mount Gagazet, they heard a song echoing across the snowy slopes."
""'Tis a song from an otherworld," they said. The soldiers panicked and ran."
"And then, as if to pursue the retreating armies, Sin appeared!"
"Some time later, scouts from Bevelle braved the mountain."
"On the other side, they witnessed the ruins that had been Zanarkand."
"The city destroyed. Not a single soul left standing. Gone!"
"In its place, a multitude of the fayth had gathered on Gagazet."
"They were singing a song."
"It's the song we now call the "Hymn of the Fayth.""
"And that, as they say, is that."
"Well...maybe not all of it."

"Rumors flew in Bevelle about Sin's sudden appearance."
"They said that the people of Zanarkand became the fayth, that they had called Sin."
"And that the man responsible..."
"was none other than the summoner Yevon, ruler of Zanarkand!"
"Yes, the lord father of Lady Yunalesca."
"On the eve of Zanarkand's destruction, Lady Yunalesca..."
"had fled to safety with her husband, Zaon."
"Later, the two used the Final Summoning to defeat Sin."
"Yet the people of Bevelle still feared Yu Yevon."
"It was to quell his wrath that they revered him, and first spread his teachings."
"And so were born the temples of Yevon."
"I suppose it's possible Yunalesca had planned it that way from the start!"
"A fair trade, she defeats Sin in exchange for her lord father's honor."
"Of course, there's no proof. No, the facts are lost in the mists of time."
"And who'd admit Yevon was an enemy of Bevelle?"
"You can bet the temples had a hand in covering that one up!"
"And that, as they say, is that."

Plenty of survivors.

Jecht didn't do it on purpose. He went out for a swim one day, went too far out, and was caught in the tide. Sin then brought him to the mainland.

Before you ask, I'm not doing a calculation of the tides of Spira and how far he would have had to swim before he could not physically swim back. If you want an answer for that, test it yourself. You can download that app that makes your phone waterproof if you want to let us know how it goes.

"I was in a coffee shop, running away from home when I heard the news."
"Our hero, Jecht, gone, vanished into thin air!"

Yuna
"A man named Jecht told me. He was my father's guardian."

[Tidus looks surprised, then slightly pissed, and turns around, muttering "Jecht" in a low voice several times]

Tidus
"My father...his name is Jecht!"

Yuna
"Amazing!"
[She does the prayer gesture]
"You know, our meeting like this must be the blessing of Yevon!"

Tidus
"Sounds like him, but it can't be him."

Yuna
"Why not?"

Tidus
"My old man, he died. Ten years ago, off the coast of Zanarkand."

[Yuna turns away]

Yuna
"I'm sorry."

Tidus
"He went out to sea for training one day...and never came back."
"And no one's seen him since then."

Yuna
"Why, that's the day that Jecht came to Spira."
"It's true! I first met Jecht ten years and three months ago!"
"I remember, that was the day my father left."
"The date fits, doesn't it?"

Reference:
http://www.ffwa.org/ff10/script.php?page=p4-02

http://www.ffwa.org/ff10/script.php?page=p1-01
http://www.ffwa.org/ff10/script.php?page=p1-04