When the crew learn that the ship is set to automatically return to Earth they are clearly disturbed by this. What is Earth like at this time in the Alien story? What has happened to Earth to make them so afraid of returning there?
In the film, Johner says, "Earth? I'm not going to that fucking slum."
A deleted final scene actually shows us the state of the planet - and it doesn't look pretty. It would appear that a cataclysmic event has left the planet a barren wasteland.
I, too, would be reluctant to return to Earth.