Why are all of these set on mining colonies (save for the first short)? Why are the leads all female?
The brief for the shorts said to use elements from the original film -- strong female lead and "working class" characters.
Why do they all include a chestburster?
Specimen doesn't.
Why do they all end up being "character creeps around in the dark with a facehugger/xeno lurking in the shadows"?
Because this basically describes every Alien movie.
You're right in that they meet the brief - I'd read the brief a while ago and thought they were more suggestions/prompts rather than hard guidelines.
It doesn't excuse the lack of originality though. Aside from the first short (Containment - which is still the worst one), all of the characters and locations are basically interchangeable and essentially the same story (character finds something suspicious and/or chestburster/facehugger, investigates, jump scare).
Don't care if I get attacked for criticising these, they aren't above it just because they're on a relatively small budget for a short - like I said, there is no excusing a lack of originality. This was a real opportunity to give fans something unique and different, perhaps take a risk or two.
Perhaps the original brief was too strict and Fox played it too safe, I don't know.
