Teacher: Why do you say that?

老师：为什么这么说？

youdao

Mike: Why do you say that?

迈克：你为什么那么说？

youdao

Jane: Why do you say that?

简：为什么这么说？

youdao

You can say "Why don't you" when you want to suggest that somebody else do something.

当你想建议他人做某事时，你可以说“Why don't you~?”。

Why don't 课堂 - SpeakingMax英语口语达人

If there are people that are actually, as you say, having a misinformation campaign, why would they do that?

NPR: Nearly Half The Country Doesn't Know Health Law Exists

Why do that when you can lie and say you make six or seven figures, and then reap whatever benefits come to someone whose data file represents affluence?

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Some will say: 'Why can you not do that AND deliver tax cuts?

BBC: NEWS | UK | Politics | Text of Michael Howard's speech