Because when you tell someone that something they hold dearly about themselves is wrong, or nonsense, or otherwise dismiss it, you're attacking who they are. You aren't attacking a thing they believe, even if you think you are. You're attacking something at their core.
It's honestly not any different than telling, say, a black person they're wrong about some race-related challenge that they live with every day. You can cite all the reputable and contrary sources that you want. It won't matter because what they live and experience every single day of their lives tells another story.
Of course there is nothing wrong with disagreeing and stating why. It would be disingenuous of you to do otherwise.