Perhaps the Alien was also blocking her way while he was busy killing Parker. Besides. Even if she did run, you saw how quickly it whupped Parker with its tail as he was trying to attack it. Then while the Alien was walking towards her, the Alien was pretty much in the position to atleast grab her if she ran. And like what a few others said, there are times when you're so scared, you can't even run. Did Ridley Scott, H.R. Giger, or any other guy working on the film actually say this was a rape scene? I honestly just heard it from people here.