 Why don't the Altairians leave Altair I if it is so overpopulated?
Interplanetary ruling. I mean, all they had to do was get a man over here, get the extradition papers signed, and provide transportation off the planet for me. I've got three million credits sitting in the Terran Consulate on Altair V, just waiting for me to walk in and pick them up. You walk into a newly opened planet with your smart little bag of tricks, walk in here with a shaky passport and no permit, with no knowledge of the natives outside of two paragraphs of inaccuracies in the Explorer's Guide, and even then you're not content to come in and sell something legitimate, something the natives might conceivably be able to use. No, nothing so simple for you. The Altairians are sore about it. And the Terran Consulate isn't willing to sell all the trading possibilities here down the river just to get you out of a mess. They spread out from Terra with the first wave of exploration—the slick, fast-talking con-men who could work new territories unfettered by the legal restrictions that soon closed down the more established planets. The first men in were the richest out, and through some curious quirk of the Terrestrial mind, they knew they could count on Terran protection, however crooked and underhand their methods. 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. You'd have found out that they're marsupials, and that two out of five of them get thrown out of their mother's pouch before they're old enough to survive. You've probably never heard that there are just too many Altairians here for the food their planet can supply, and their diet is so finicky that they just can't live on anything that doesn't grow here. They've built up the most confusing and impossible system of barter and trade imaginable, aimed at individual survival, with land as the value behind the credit. They have only the haziest idea what truth represents, and they've shrugged off the idea as impossible and useless." Encroachment on private property is legal grounds for murder on this planet, and twelve of them descended on the same chunk of land at the same time, all armed with title-deeds." "You've got twelve mad Altairians in your hair.
 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.