Why are people afraid to speak plainly to their doctors?

I recently read a comment that baffled me. Someone explaining about seeking more information said "I wouldn't be rude & insult her practice"... Why on earth would it be "rude or insulting" to tell your doctor that you've researched the medication and you're not interested in taking it? Speak up to your doctor --- lord knows a lot of them need to know that they don't know everything.