Because "white knighting" defies the idea of discussion by simply discarding any justified criticism.
If we had only "white knights" in this forum it would not be discussion but "praise anything we got served on the table".
But the assumption that opposing opinions are automatically White Knights precludes constructive conversation.
Comments like "White Knights incoming." implies that their mind is already made up on the subject and they're going to disregard anything that they disagree with.
The art of good discourse is to always be open to the fact that you may be wrong.