i think a combination of the monster, the others, a general fear of going too far into the jungle and the fact that they aren't very easy to find is why they havent found any others. And i dont think desmond could have found another hatch since he got out very little, even if he ever got out at all.

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\"We\'re not alone on this Island and we all KNOW IT\" - John Locke

Locke: Why do you find it so hard to believe?
Jack: Why do you find it so easy?
Locke: It\'s never been easy.

Desmond seemed so paranoid about entering the numbers every 108 minutes that I don't think he'd have gone outside at all. He probably feared something happening to keep him from returning in time. Also, Locke made mention about the lights providing artificial sunlight (I presume for health reasons) so it appears that whoever planned the station didn't intend that the button pushers leave.

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Things fall apart, the center cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;

--Lines 2-6, "The Second Coming" by Wm. Butler Yeats