XD I doubt this is it, but...
Perhaps because actual space travel would be...nearly impossible?
For *so* many reasons, the people of Earth (in reality) are currently limited to *Earth* as far as travel goes.
First, radiation--outside of our solar system, there's no protection against cosmic radiation. That means, unless you got yourself some skippy shielding? You have brain cancer after about five minutes.
Second, heat--XD Nebulae and other stars alike are way way way hotter than even the sun--heck, the Io Torus is the hottest place in our Solar system, that's around Jupiter an' all...so it'd be a matter of finding a ship that could fly through the space gases without burning up...which, shall we say, is *very* unlikely. ( ^_^;; only thing not burning in a temperature like that would be either some miraculously stable element that has a very dense molecular structure that we haven't found yet or...well, that's about it XD;;; )
Third? Time/Distance. You can only travel up to about the speed of Light. 'Less you're Flash, then presumably you can cross it. That leaves the world with very *sloooww* means of space travel...only *approaching* the speed of light, not even necessarily reaching it. That means it takes a *long* time to get anywhere.
XD so basically? I figure it's probably just way too much trouble for the people of earth to get anywhere with space travel. It's almost unfeasible.
But. That's assuming the writers of the show know astronomy and all and have chosen to use those facts XD; instead of something else.
So...that kinda answers your question...assuming that all the basic laws of physics, yadda, are in operation in the JL universe. Which...granted, they probably aren't, because the Javelin exists. XD; )
"Hello, Children!" He called, and Irvine, Zell, and Quistis screeched to a halt, thwarted in their attempt to sneak by. "Isn't this the most fantastic day? How are all of you?"
"Fine." Quistis said.
"Great," Zell said.
"Good," Irvine said.
"Mrummph," said Irvine's coat and hat, which the three SeeDs seemed to be carrying between them, and wiggled purple striped toes.
~THE WEDDING FIC (Tenshi no Korin, Bishonenink)