Can't it be "why are you not eating the vegetable?" ? it appears that the Present Progressive form in English is much more suitable than the Present Simple form. If the sentence was a general statement about vegetables, then I could understand, but the noun "vegetable" has a definite article...

The first time I saw this "application" I had written veggies and got marked down. I was like "okay, next time I'll use vegetables or greens as indicated by the accepted responses." Well, now I used greens and I got marked down again! WTH Duolingo!

This is not correct, but is accepted as a correct response: Why do not you eat vegetables?