Asking them to assign themselves grades, would challenge the dominant power of teacher as the almighty transmitter of knowledge which is consumed by students and scribed into a grade.
I think students are very much aware of this power. That is often why we under-grade ourselves when given the option to self grade. Even though we know we put in the effort and time we still feel as though we don't have the authority to give ourselves that A. In our minds we may be thinking “only a teacher could be truly unbiased and determine if this is A work or not”. That is why also having classmates grade you as well is good. They may confirm what the teacher thinks of your work. If your group members are giving you A’s as well, perhaps that boosts your confidence when you self-grade the next time.