Re: The Mystery of Athas. A plausible reason.

You seem to forget that Athas is in a different dimensional plane and therefore
cannot be acessed by spelljamming.
This is written in the Dragon Kings sourcebook and the sourcebook about
preservers
and defilers.
By way unknown Athas is on an alternate prime material plane which makes
palanar travel very difficult. The world is surrounded by the demiplane of
the grey which is similar to a demiplane of dead because the souls of all dead
athasians are sucked into this plane and dissipate there. All beings travelling
through this plane to and from athas are slowly dissipating, too. And if
they do not find a way out they die there.

I hope this helps!

Bjoern