Essentially you asked the question with a pre-conceived notion of what the answer would be and it sounds like you're disappointed you didn't get the answer you wanted.
Different people have different reasons for seeing movies and enjoy them for different reasons. One thing that makes movies different from books is that movies have time limits and pacing is a much tougher issue for them than for books. In the case of your question, one of the answers talked about a deleted scene and builds a strong case for their answer. I know I learned something from that answer about a classic SF/horror film. And that's a major point of asking a question: Often you get an answer where you learn something totally unexpected.
Now, as to your direct question (the previous point was more about issues around your question here), there's the willing suspension of disbelief and the bar is higher on this for some than others. For many people, if the answer to your question were, "It's a plot device," then that kills it and kills their ease at suspending their disbelief. Allowing writers and directors to just insert plot devices make the here unbeatable, and then there's no thrill to the story. If the character is not vulnerable and not in danger, then where's the suspense?
If we know every time Ripley is in trouble, the alien will conveniently be asleep or just not notice her, then it makes the story line far less believable for many and makes it harder to believe the film.
Both science fiction and fantasy have a high level of escapism built into them as genres. Many of us, when watching Aliens, are thinking we're Ripley, out in space, in that ship, and facing that danger (or we're Luke Skywalker or Jean-Luc Picard or Lara Croft or any other hero). If, when something comes up, you have to suddenly break the fourth wall and say, "Okay, this isn't really believable, so we'll just say the alien is sleeping at this point," then we are no longer on a shuttle escaping from the Nostromo, but we're on a set looking at cameras and lights and a film crew or we're in the theater with Betsy-Sue Frederickson again, upset because we long since realized we're not getting to second base with her on this date because the movie isn't scaring her enough to grab on to us.
"It's a plot device," is always a fallback answer, but often we find that the universe created in a movie or book is far more complex than we imagined. For instance, in Aliens, we saw much more of the life-cycle of the alien (which essentially answered your question -- there's no need to eat EVERY tender morsel you encounter, some you need to use to incubate your young). I'm amazed at the amount of explanation provided for something I may ask about in Star Wars that seems like a plot hole or oversight by George Lucas. Often the original issue is an oversight, but sometimes something quite interesting comes out of the retconning or explanations to justify what was originally an error.
And, as I said, on the other hand, often we don't want to have our willing suspension of disbelief broken because characters do something that is only for the convenience of telling the story at the expense of believability.
