That wasn’t the situation they started out in. Ripley had originally told her daughter that she would be back in time for her birthday. So the crew must have expected to only be on the ship for a few months.
It wasn’t until Ripley had to finish the voyage in the lifeboat that it turned into a trip lasting decades. And if I recall correctly, even that was unexpected. I think there was a scene where they said the lifeboat had malfunctioned and become lost.
Her… [sob]… eleventh… birthday…
I think Burke said her lifeboat “drifted through the core systems” (the mechanics of FTL travel are never clearly addressed, but I can forgive that) and her recovery was by chance. I think when she makes her final log entry at the end of Alien, she says “With a little luck…” she’d be quickly found, suggesting it was a tad risky.
Watch carefully the scene when the alien jumps out as the captain looks for it in some kind of dark pipe/tunnel.
That scene almost made me wet myself when it was released.
Years later I watched it again, this time in super-slow motion with pauses.
It seems to be a skinny dancer wearing a painted leotard throwing his skinny arms out to the camera, “Ta-Daa!”.
Which reminds me to rewatch it tonight on my new HD TV. 
Jazz fangs!