"Why aren't you studying right now?"
Translation:¿Por qué no estás estudiando ahora mismo?
Duolingo lessons hardly ever are intended to teach only one thing. Duo has algorithms and methods to teach many thing simultaneously including vocabulary, repeated vocabulary, grammar, sentence structure, idioms, spelling, verb conjugation, nuance, culture, and much more.
Therefore, it makes a lot of sense to include a time phrase in these sentences in order to teach that vocabulary and learn how a native might speak.
I know that in English we do say both "Why aren't you studying right now?" AND "Why aren't you studying?" The first phrase could convey a nuance of forcefulness (possibly an authority figure) and is very immediate (while looking at the person in this instant). The latter could mean the same, depending on voice inflection, or it might be more general, such as a time period that involves this week or even this semester - as when a student is asking another student if they are keeping up in class. So, the sentences might be used in different settings or the same. It depends.