Well, it is also observably untrue which is the problem with statements like these. "Some girls are not got at throwing a ball" or "You throw like a girl who is not good at throwing a ball" would be more correct statements, but especially the second one is rather redundant. The problem with "girls can't throw" is that it becomes a sort of self-fulfilling prophecy. If you're told over and over again that you can't throw that's going to affect you negatively.
In that case it might be less controversial to say "some girls can't throw because my country does not properly educate them in that field".
Yes, they are arbitrary and ridiculous, but they do harm to those who break away from those norms. Girls with short hair who wear jeans or other clothes that are associated with boys are going to have a harder time than those who confrom to the stereotype. And boys have it even harder with this than girls. Imagine a boy who has long hair, wears make-up and a dress, because he thinks that it looks good on him. He's not transgender, he's just a boy who wears clothes that our society associates with the female gender. He's going to have a very hard time because of over-generalized statementes like "girls have long hair, boy's don't".
Because said quality is negative? If you're saying "You throw like a girl" then you're assigning a bad quality with a gender which in turn casts said gender in a bad light when that quality is concerned.
Well, you're sort of right about boys; them "reeking" is not a societal wrong, but girls being told that they're bad at throwing because they're girls, a group that's bad at throwing is a societal wrong. And not saying things like "You throw like a girl" helps stop the spreading of such wrongs.