A: どうしてスイカを食べませんか。
B: スイカが好きではありません。
Though it is unnatural, textbook Japanese,
it makes sense, has no problem grammatically, you are speaking what you mean.
>because it's like to say "do you want watermelon"
>どうしてスイカを食べませんか。
This Japanese asks the reason why you don't eat water melon (wih slight accusing touch).
I don't think the sentence means "do you want watermelon?" in Japanese.