I don't think gender assigned at birth directly makes any assertions of gender roles. It's simply a doctor saying "you have these genitals, therefore you are this gender".

Gender roles are a societal problem; not a facet of what the doctor put on your birth certificate.

Also, the terms "AMAB" and "AFAB" are descriptive; this is literally what is put on the birth certificate.

We can't say "assigned boy at birth" because that is not what happens; you are assigned "male" at birth.