Seriously, understanding the "real answer" is a combination of lack of imagination plus producers' concerns about the viewing audience not "getting" an alien shaped like a sponge or something ...
is there in fact some advantage to being bipedal? Like offering better mobility without the metabolic cost of additional limbs?
The aliens' faces in this film's preview, though, look unforgivably humanoid IMHO
Why are all aliens bipeds? Dunno - maybe Jabba the Hutt could tell us.
shareHave you ever seen an alien with more than two legs? I haven't
share<<why are all aliens bipeds?>>
--Because they have two feet?????
Because we're a primitive and superstitious species, we tend anthropomorphise the unknown & unknowable in our own image. Not just angels, but demons too. The gods (both good & bad) were always traditionally humanoid, as are goblins, dwarves, fairies, elves, trolls, leprechauns, demons, and space-aliens. All sufficiently humanoid to be percieved as a threat, whilst also sufficiently different to be ruthlessly eliminated.
When a primitive intelligence such as ours seeks to wage war against others, we tend to "demonise" or highlight the differences between 'us' & 'them'. Repetitious demonisation will eventually allow the suspension of opposition to what otherwise might be perceived as needlessly aggressive behaviour. Similar behaviour may be seen in chimps (our closest living genetic relatives) when they engage in wars of annihilation against neighbouring clans, and will wage war until the last rival is murdered. Look at the rhetoric coming from the White house prior to the invasions of the small south asian countries in the early 21st century. Babies being tipped out of Kuwaiti humidicribs and crushed underfoot by "eyewitnesses", just like the Hun supposedly bayoneting Belgian babies in 1914. The more barbaric we can make these "others", the more barbaric and "uncivilised" our response is justified in being: this is how torture is justified.
This erroneous justification loop is alive and well within the rhetoric of the current pretenders to the throne of US politics. The "threat" from barbaric, uncivilised baby-killing "terrorists", foreigners and other undesirables justifies inhumane treatment of refugees, exclusion of "undesirable" races, suspension and elimination of hard won civil liberties, and continuing acceptance of civil and administrative corruption on an unprecedented scale. It's also much, much easier to control a population, and to justify the reduction of civil liberties in a population that feels threatened.
Just substitute the interchangeable concepts of space-alien for terrorist-alien and you get the gist of my assertion.