Because visual art is a different medium from fiction. It’s not apples to apples. Everything in visual art is an aesthetic. People read fiction to get inside the heads and experiences and worlds of characters; it’s not quite the same as a picture or even storytelling via a graphic novel or comic.
At the risk of repeating what others have already said – my question back to “who cares if it’s just references to ears and tails,” is, why write a nonhuman character and then have their nonhumanness play no role in the story? It’s not like having human characters of various races or genders, where you could argue that you’re representing the diversity that’s realistically present in human life. Why make a character an alien, or a vampire, or half-elf, or a German shepherd unless it influences the character and/or the story in some way?
(As an aside, I know I’ve been misinterpreted on this point by others before, so I’m just going to stop here and explain: My preference in furry fiction – reading and writing – is for stories where the anthro nature of the characters adds something to the story. That’s not the same as saying it has to be essential to the plot. Sometimes it can be as simple as adding a fable-like feeling to the story, even if the plot would work with humans. I also don’t think you have to explain where the anthros came from, unless that’s somehow important to the story you’re telling. As an egotistic example, I never explained where the anthro cats of “Best of Breed” came from, and personally I think it still works just fine – and still couldn’t be told with humans while remaining the same story.)
At this point, all that's been done is saying "I don't think you're writing to my standards."
I’m with Voice on this one. I’m not saying it isn’t furry when it’s just animal costumes. I am saying it’s not necessarily furry literature at its best – which in this case I’d define as, most creative and reaching its fullest potential.
If the consensus is that furry is a genre (or insert whatever word best fits), then naturally, as with any other genre, there are going to be works that push the envelope and aim for using anthropomorphics (the key element that makes furry fiction furry fiction) as something more than just window dressing, to explore aspects of character and world and story that can’t be easily explored another way, and then there are going to be works that do use it just as window dressing.* And there are plenty of readers in the fandom who are perfectly fine with what I call the fox-in-Starbucks stories that are just our regular human world with animal heads on everybody, where the story wouldn’t change a bit with humans besides having to edit out minor references to muzzles and tails and such. Just like there are readers who are fine with putting a character on a spaceship and calling it science fiction, when maybe the story could just as easily take place in the middle of Nebraska. And then there are other readers who are pickier and want more from whatever their preferred genre is.
So, as far as why an author should care, I guess it boils down to what audience they’re writing for.
*(And to be fair, there’s probably kind of a spectrum here; it’s not just nonessential vs essential – the same way there’s a spectrum from very animal-like characters to very human-like characters.)