 Why don't the Altairians leave Altair I if it is so overpopulated?
Altair I had been recognized at once by the Trading Commission as a commercial prize of tremendous value, but early reports had warned of the danger of wildcat trading on the little, musty, jungle-like planet with its shaggy, three-eyed inhabitants—warned specifically against the confidence tactics so frequently used—but there was always somebody, Meyerhoff reflected sourly, who just didn't get the word. Loud, harsh voices were heard as half a dozen of the huge Altairians attempted to push through the door at once. Finally the question of precedent seemed to be settled, and a group of the Altairians filed in, in order of stature, stalking across the room in flowing black robes, pug-nosed faces glowering with self-importance.
 They don't leave because they can only eat food grown on Altair I.
 They don't leave because they can only eat food grown on Altair I.  The Goddess won't let them leave.  They don't leave because no other planets will clear ships from Altair I for landing. Nobody likes liars.  They don't leave because they have not achieved space travel.