Humanity has colonized the galaxy and have strict quarantine between world's, Ripley mentioned it in passing. First there's the fear of alien bacteria n stuff n there's has to have been some outbreaks considering the sheer amount of systems in the galaxy we colonized in a short time. Considering humanity is still around n thriving they must have overcome bio threats n won't be caught unprepared.
Even if Xenomorphs could somehow infect one world, they couldn't do it against all our worlds with humanity unawares.
2nd our armies would be better n with robots n drones n stuff, the xenos would hit a wall.
Humans despite their faults are better survivors than xenos, only in movies could we be threatened, unprepared, undergun'd n stupidity exists to drive the story forwards...
I would be more afraid of the Predators who have all the tools to overcome humanity..stronger, very intelligent n better technology.
Xenomorphs lack all the tools needed to tip the scales..
The Xenomorphs would need to at least be at the same level as the Zerg to be a threat (Zerg would eat xenomorphs alive) those (Zerg) were overblown as well (no defense against long range weapons n nukes).
Predators choose not to wipe us out even though they could, the Protoss as well, isn't that true supremacy? Feeling secure in their power.
Xenomorphs though kill whatever's in front of them with no vision n always feeling everything is a threat to them. They are limited...