Because when you have an intergalactic race of killing machines, it only makes sense to keep coming back to the same small, incestuous circle of protagonists.
Anyway, on the heels of a disastrously received Aliens game about Colonial Marines (albeit one with cool tie-in toys), Sega appears to be going for the publicity stunt with Alien: Isolation, a new game that will reveal that between her pre-teenhood around the time of Alien and death prior to Aliens, Amanda Ripley encountered mom's familiar monster while working on a space station. There's only one xenomorph for most of the game, apparently, which doesn't sound very fun - you can shoot at soldiers and clones, but if history has told us anything, it's that an attack of the clones bores fanboys pretty quickly.
Now, if Amanda's supposed to die of old age, but you let her die from an alien attack here - aren't you screwing up the time stream? We mostly leave those dilemmas to Time Lords. Mostly.