suggestion: dispute button for "wrong" answers
The test question I was given asked for a translation of an English sentence using a form of "to be". In many languages, and in many informal settings, a valid translation may leave it out. But in this language it was possible to give a formal answer with "to be" included. I understood the "correct" answer, but my answer was not "wrong".
I wanted to press DISPUTE, to add a valid translation to this sentence, in this context.
This is on iOS.
While I understand where you are coming from, I believe, we are here to learn the "correct" language. So while you may be sure that your answer without the verb is acceptable, some other learner may leave it out just because they don't know the correct form, yet their answer will be accepted and they thus won't have chance to learn the conventional form in the first place. I think Duolingo's being rather conservative here is a good thing.
What quest42good says is especially true because Duo uses computers to run the whole system.
In French, some students want to move phrases from one position in the sentence to another when it isn't necessary. When it is marked wrong, they object by pointing out that it just as good a translation as the Duo answer.
For Duo, programing in all the possible arrangements of words and phrases would increase the complexity of the programming exponentially. In the end, they still couldn't be sure if the student truly understood the language.
The alternative system that would allow students to change words or structure because it appears to mean the same, would be to rely on using humans to read or listen to answers. Much better but infinitely more expensive.