"Are you all not eating?"
the solution 你们不吃饭吗？contains 饭 = rice, why is it not "do you eat rice"?
From my study, Rice is so associated with eating that 吃饭 is eat/eating... Hope this helps.
I think they use "you all" to differentiate the plural "you" from the singular. Just using "you" wouldn't be explicit.
I used 都 as well. I think it's just not what they want us to learn in this lesson.
Rice is heavily associated with food in the Chinese culture. 饭 usually refers to a meal.