Over the years, many people engaged on the Alien franchise had spoken about the need to explain the aliens - where they came from, how they were made, why they were so 'hostile', what they wanted. That was always an understandable mistake. As both figures on the screen and entities in a story, they had a magnificence, an arbitrariness, that would have been spoiled by explanation. Far more important than any causative, narrative answer to how they had acid for blood was their living up to the legendary status of being endlessly renewable, nasty, dangerous and beyond reason. To be alien is to be unknowable.
David Thomson on the Alien Quartet, p. 171 (Bloomsbury, London: 1998)