It's exactly the same reason why masked heroes have to always appear "unmasked" for x-amount of the movie, audiences NEED to relate to facial expressions.
V himself was masked but Natalie Portman's character was essentially the "proxy" for his humanity. She represented the moral compass of V.
Anyone who seriously thinks that the "average joe" would be interested in a 2-hour movie where literally every character is a snarling animal is on one.
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"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."
"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."
I think wearing a mask as a superhero does trying to keep his true identity a secret is a bit different from only having humanoid monsters and NO humans in a film. To have a story there has to be some sort of human element. It would be pretty experimental to eliminate humans all together.
Edit: I should add that they've touched on this in just about every film with a Predator(s); that they're not just "snarling animals". Predator showed us that they have a code on honor, Predator 2 did the same with the elder Pred. AVP expanded a little more. Predators again with the captured Yautja. Character development could be done and if done well, would work. Even the Xenomorphs where shown to have intelligence in Alien: Resurrection.
Except for Capt. America, the "norm" was keeping their identities a secret & yet they either always took their masks off for some lame reason OR it was the only part of their costume that got damaged/ ripped off.