Yeah people think government is like, you elect people and they get a turn at the wheel. But all the people you could elect aren't equally experienced or competent drivers, and the car doesn't suit them all equally (this is simple systems theory, either your government is highly tailored towards--say--centrists running it and thus is very efficient, or anyone on any part of the political spectrum could run it but it's significantly less efficient).
Or like, in your example (to be clear I defer to you entirely on the facts), not only would Foxx have to be a competent State's Attorney, she'd have to be competent at changing a system that's not ideal for her into one that is, e.g. by persuading existing staff, hiring new staff, etc.
This is by no means impossible--it's called leadership. My hot take, more or less, is that if you get someone into office who shares your values but can't make the office execute those values, not only will you fail but you'll tarnish your political movement for... a good long time, so competence is actually more important than values alignment.
My even hotter take is that people used to know this. When people from minority groups or political persuasions got power (women, LGBTQ people, racial/ethnic/religious minorities, ideological/philosophical minorities, etc) they knew they were modeling for everyone in their group. Is it fair? No. Is it reality. Super yes! Is whining about unfairness good politics and/or attractive at all? Super no!