"Why do you not eat fruit?"
Translation:Por que você não come fruta?
In this case there is a general sense of the fruits that the second person does not eat. In english it's ok to use the singular (fruit) but in portuguese I'm sure you'd have to use the plural. It could be ambiguous or incomplete to say "fruta" although it is understandable. But "frutas" to me sounds more appropriate
Por que você não come fruta?
"Why don't you eat fruit?" "Why do you not eat fruit?"
Not sure if one of this options is used more commonly in spoken English. The other way around in spoken portuguese the "por que" would be often omitted to simplify the language.