Unless theyre super Alien fanatics, I have to assume that most artists understandably have no idea how to draw an Alien. Yeah, they remember the creature generally from watching a movie or two a few times in their life, but its architecture is not at all simplistic and something often hidden in shadow, so much so that to draw it, they are going to need to reference photos, figures or something... and draw it often enough... before they are able to commit it to memory.