That would require rewriting the miis from scratch. When they were first created miis had binary genders, and that assumption was baked in from the start. Changing it now would be expensive so they do this sort of half measure.

A lot of games tried to retrofit gender diversity into already existing character systems and it doesn't work well. For example, cyberpunk's V can have any genitals or body shape you want, but the game was written from the start with an assumed strict gender binary. Your character's gender is whichever voice you pick.

Hopefully in the future these character systems will be built with that sort of flexibility. But its quite common to have strange results from retrofits like the miis.