If you ever decide to turn yourself into a gaming flagellant and punish yourself for all that sinful, fun gaming you've been doing, you could always purchase one of Sega's Aliens titles. Or at least, you could until recently, as both the last Aliens vs Predator and Aliens: Colonial Marines games have been pulled from Steam.
As much as it would be fun to pretend it's because they weren't very good (they weren't) it's more likely that the license for either Valve or Sega to sell them has now run its course, so the games are no longer available on the digital distribution platform.
This is the same sort of thing that would see high-street retailers stop selling a game (beyond the fact that people stop buying them after a while) but is far more obvious because the digital shelves of Steam are far vaster than those of any real world gaming store.
If you really want to play either of them, they're still available in physical form from some outlets and you can no doubt find some CD keys of them somewhere. However, if you just want to play a game in the Alien universe, Isolation is far superior to both - although very different, with a focus on survival horror, rather than action.