Translation:Why are you always looking at your phone?
We don't watch our phones, just look at or stare at them. I know it seems strange, but that's just the chosen verb combo in English.
"Why are you always on your mobile phone" is refused. I am not english speaker.
Your translation implies that you're making a phone call. This is not accurate.
Since this is a course for English speakers learning Chinese, the more colloquial "Why are you always on your phone?" should be accepted. That's the way an English speaker would say that anyways.
You are correct that the Chinese sentence can be translated TO English in this way. When you do it backwards, i.e. translate "Why are you always on your phone?" back to Chinese, there would be more interpretations like 看手机 / 玩手机 / 對著手机
"why are you looking at your phone all the time" is perfectly correct... Fix your system please if you want people to continue using Duolingo...
"Why are you looking at your phone all the time" is perfectly correct. Wake up!
"Why are you looking at your phone all the time" is perfectly correct. This Chinese course has many problems, and no one knows when you will those problems...