Ela já está impregnada, então não há necessidade.
Newt, em Aliens , é encapsulada para prepará-la para qualquer facehugger nas proximidades.
O período de gestação é de 24 horas, então por que se incomodar? A maioria das espécies sencientes não está inclinada a se auto-sacrificar, e qualquer subespécie não faria isso.
Está funcionando normalmente até que mamãe chega em casa:
Why did the alien kill everyone instead of cocooning them?
In the original film, the alien birthed from Kane (John Hurt), killed Parker (Yaphet Kotto) and Lambert (Veronica Cartwright), and cocooned only Brett (Harry Dean Stanton) and Dallas (Tom Skerritt) although, in the continuity of the series, their deaths are left ambiguous.
In Aliens, they killed the majority of the Marines, but they cocooned Apone (Al Matthews), Dietrich (Cynthia Dale Scott), and possibly Hudson (Bill Paxton) and Burke (Paul Reiser) (the original script and novelisation of the second film included a scene where Ripley finds Burke cocooned). A large majority of the colonists were cocooned as well.
It was about 15 people vs 100 aliens. In this film, it is one Alien versus approximately 25 people, so it needs to get rid of a lot of humans that could potentially become a threat if they work together. –IMDB