Every person in these movies is a blistering idiot. Like, the levels of stupid on every single person in these expeditions is insane.
The original Alien didn’t require the entire cast to be complete morons, unfit to even operate as an adult in society, much less be mission-critical crew on a colonization mission.
If it hadn’t been for the android in the original Alien, they would have observed reasonable quarantine procedures and there wouldn’t really have been a problem.
That’s one of the main reasons I think these movies are bad. The writers either did not care enough or were too incompetent to write intelligent, believable, relatable characters that I actually grow to care about.
Consider this question: how is it that anyone under the age of 40 today has ever smoked?
By the time they were born, the bad effects of smoking were well understood. By the time they were teenagers, not smoking should have been as obvious as not jumping in front of a train. People already addicted find it difficult to quit, but it in no way explains anyone starting.
The question is different and yet very similar, because the things you mention wind up in a similar way. Somehow people start in that route even though it should be obvious not to. And these things you mention are much easier to fall into than smoking because parents, family, etc are all pushing it on people. Smokers generally aren’t pushing their kids, nieces and nephews, grandchildren, etc to smoke, and somehow smoking still proliferates to some degree, just consider how much more difficult to avoid it is for those whose families are actively encouraging them to fall into these methods of belief and hate.