Well, it's a correct answer, so what's the problem? The main English sentence is "Why are you not eating the rice?", actually. With "Why do you not eat rice?" as the second starred version.
Here's a thought: Most English speakers would say - why aren't you eating rice. No one uses such archaic sentence construction.
That archaic construction is "why are you not"? Yeah, true, but apart from really rare situations we don't put the answers with contractions like that. That's problematic when the app creates tiles for some exercises.
I thought about putting 'why do you not eat rice?' but didn't because the hints said 'are not eating'. the English sentences mean something completely different. 'Why do you not eat?' means that you never do & 'why are you not eating?' means -'What's wrong with the rice? why aren't you eating it? Is there no differentiation in Polish?
There can be, but there doesn't have to be. Most Polish verbs don't show any difference between English Present Simple and Present Continuous. The exceptions are Verbs of Motion and the idea of 'wearing' something.
But some verbs have also their 'habitual' equivalents. Those are not obligatory, so to say, but they are useful. They make it clear that the sentence should be translated into Present Simple.
So "Dlaczego nie jesz ryżu?" may be translated both as "aren't you eating" and "do you not eat", and "aren't you eating" seems more probable, but "do you not eat" is still correct.
But you can also say "Dlaczego nie jadasz ryżu?" (the verb is "jadać"), and that is definitely "Why do you not eat rice?".
The habitual verbs aren't taught in this course.