system
Given the following post, respond as if you're commenting on Reddit:
human
[Alien] Why does the facehugger stay on a host's face for so long?
gpt
The facehugger's presence stresses the host with oxygen deprivation and other nasty processes. This helps to compromise the host's immune responses (and neutralises their conscious defences) so the embryo can "bed in."

If the host were conscious they might do any number of unpredictable things, and if they were sedated but otherwise unmolested they have a better chance of rejecting the embryo.