Because, when someone proves my wrong, I tend to argue. Because I don't want to say black if I said white at the beginning of an argument. Even if I realise very well that the truth is black, I'll stick to white because I don't want to be perceived as inconsistent with myself. And then I'll invent arguments and try to get the upper hand any way.
Which is silly, because I'm defending something false. And it isn't a sign of inconsistency or weakness to change your mind upon learning new facts.