To answer the question posed in the subject: The reason is “setting.” Those character traits are the setting of Burning Wheel.
To answer a question posed in the original post: “Shouldn’t I be free to choose my character history…?” No. Freedom is an illusion in games. Choice is always constrained. In this case, we constrain your choice to represent the setting and to encourage you to make meaningful, difficult, impactful choices about the character and the world without asking you to read histories or metaplots or even asking you to have played the game yet.