I think in alien its excusable because its a mainly non-science crew wanting to blow it out the goddamn airlock.
In Aliens, the conference room scene confirms of no lifeform like that discovered in xyz surveyed worlds. The military crew there being trained not to flinch even if a pink elephant flew across right in front of them.
Prometheus however... Not excused. . We all know that in real life, reactions of those Nasa and Esa scientists are not at all muted. So to have Prometheus scientists NOT be awestruck, or NOT bursting with excitement from every orifice over the discovery of a new alien lifeform (even if its the 10th new lifeform theyve discovered that day) is inexcusable.
It was the first thing I noticed when I watched Prometheus at the cinema first time around and the crew come across the decapitated body. No wonderment. No yelps that a posited theory had been proven or disproven. Nothing. I knew at that instant, right there and then, that the film was off in some way. A chink in the armor...