Why go to the United States? 或 Why do you go to America? 或 Why are you going to the US? 或 Why are you going to America? 都可以表达“为什么去美国”的意思，具体使用哪个句子，需要根据上下文和语境来决定。
Why go to the United States? It's so far away.（为什么去美国？它那么远。）
Why do you go to America? Is there anything special there?（你为什么去美国？那里有什么特别的吗？）
Why are you going to the US? Are you going on a business trip or for tourism?（你为什么要去美国？你是去出差还是旅游？）
Why are you going to America? I heard the scenery there is beautiful.（你为什么要去美国？我听说那里的风景很美。）