You are correct, but it's capitalism and the adrenaline kick. We are culturally conditioned folk, and all scripts about alien invasions that end up in movies follow a completely implausible scenario. Most don't even bother any longer to give a reason for invasion. I would blame the writers of those scripts, but movies without violence don't give the adrenaline kick and don't sell well.

Here is an idea for any writers reading this who want to break the violence cycle: whenever you feel you need to have a character kill another in a gratuitously violent way, twist the plot so that they instead end up having steamy sex. The readers will get an intense hormone rush that way too.