Based on a metric of producing code that fulfills the requirements, they may be as much an A player as the assertive developer down the hall. The thought of including assertiveness in measuring employee performance has never occurred to them. You say to them that only "A" players hire equally capable people, but they respond by saying that they are an "A" player. It is the start of an unreasonable argument.
Perhaps they should hire for assertiveness, but that's a matter of convincing people that their method of analysis is wrong. That doesn't get solved with platitudes; it gets solved by showing that encouraging assertiveness provide more value that the security of the status quo. How to show this, though, is something I am still trying to figure out.
More than you might imagine.
Think of all the people you've worked with, in whatever capacity, who never question requirements. It seems likely they will hire people of the same sort, not just because they might have a bias toward the "good" employee being subservient, but because someone who questions the requirements to them will inevitably want them to question the requirements to their superior. They've already spent their career up to this point avoiding this, it seems unlikely they'll take some action now that prevents it.