Because it is a truth most of us have accepted without question. Further, it is so agreeable to other facts. Left handed people are evil? But of course, they can attack up in castle staircases! Medeival life was one of constant sword-fighting and war time!

Also I'll admit the first couple arguments in TFA felt weak. I wouldn't be surprised if that colored peoples readings of the rest, or if they jumped to come comment before finishing the rest.