Other mistake: When the father and son sneak into the quarantine area, they are wearing radiation masks. The father's mask is a slip over face mask (not entire face, hood and shoulders one-piece like the son's) that shows some exposed skin in a gap between the hood and mask, yet they speak about how severe the radiation is expected to be. The father using a meter detects no radiation and he dramatically slips that mask type off to breath the air. If there was severe radiation as they anticipated, he would have already been harshly exposed.
Other mistake: In the train rail scene, we see Femuto has eggs inside her pouch, but later in the movie before she lays her eggs, there are no eggs in her pouch.
Answer: Because detaching them and reattaching them would have taken time, which they didn't have.
But, after the crash, they remove the warheads and transport them by helicopter and leave the boosters behind, as they only need the warheads.