It's the same in Alien Isolation though. Once you realize how the Alien patrols and when he shows up it's not scary anymore. Plus it was really easy to hide from the Alien. I had the opposite effect, Alien became a nuisance, Jack freaking chased me through half the house then grabbed me through the wall in front of me again, damn!
The thing is, that's all scripted. From an AI standpoint, Jack was assigned to walk in what I think I can safely call lukewarm pursuit after the player, and then once the player reached a certain point, Jack is loaded into a hiding point behind the wall and then pops out to grab the player once they walk past. Perhaps this is just me being cynical, but once I've figured out what an AI's limitations are and how to abuse them, they become significantly less frightening. With Jack, I had him figured out by the second time I ran into him upstairs.
The alien does get more predictable later on; you can certainly begin to figure out how it works (game gives it a set area within which to search and it will randomly check places unless it notices something), but I still find it more frightening because, once it's in play, it's a threat that you have to treat realistically. With Jack or Marguerite, if I get spotted, I can just run a few rooms over and they'll stop following me. With the alien, that's not an option. You have to treat it like a genuine threat whenever it's around or you will die. Jack and Marguerite's AI is much more abusable than the Alien's, and I think that itself says a lot about the level of fear that the two AIs instill.