Why do you conflate politeness and trustworthiness? Seems like a weird connection to make.
Is it really that weird? Imagine someone going to a store and the owner starts swearing at them because they asked a question. Would said visitor be more or less likely to trust the owner? I agree that being impolite doesn't necessarily equate to being ignorant in one's subject, but I wouldn't be surprised that on average the most knowledgeable and wise tend to be more polite.