[Oblivion437]

The first and second Alien movies, the first Predator movie set up certain pretenses about their respective science fiction villains. Like who the hell were those pilot creatures, you know, the big bastards who were on that ship, like the fellow in the chair with the hole in his chest? Obviously these aliens were a cargo of sorts, or else, this was where they were made. This observation is expounded upon heavily in log entries in Aliens vs. Predator 2, by various members of science teams studying them...They're engineered...Their population growth is limited by how far their parasitic reproducers can find hosts. No hosts means no new cuddly baby aliens, no food for the big burly ones, and obviously, after a time, all usable life thus destroyed, whatever they attack totally barren, they go extinct themselves. It's what the Neutron Bomb wasn't. A built in design flaw/fail safe doesn't exist in nature...The Lemmings thing is a myth...

Something important to note is that the original designer of the aliens, H.R. Giger, and his cohort and much enthusiastic colleague in bringing an existing creation to film, Ridley Scott, have had little to no involvement past Alien...