Let's imagine a hypothetical America, where we remove the economic aspect. Let's imagine that every household in america pulls in $250k/year and prices are uniform for every household across the nation. No one is poor. Everyone goes to the same excellent free schools where the teachers also make $250k/year. We've removed economics as a variable. Everyone is equally educated and equally rich.
But let's leave one variable in: America is exactly as racist as America is currently. Ethnic backgrounds and cultures still exist, and there are enough white assholes in "gatekeeping" roles to affect the distribution of people who pass their gates. This is a fact that is a true thing that already exists in America; we're not ADDING it to the model, we've just left it in as the only thing we want to measure.
Structurally, as a society, you want the distribution of people who pass through higher education into roles like "Doctors" and "Lawyers" and eventually "Politicians" to broadly match the distribution of cultures that comprise the society as a whole. Otherwise you create an apartheid state, and an angry under-class that threatens the stability of the system. This is an axiom so simple that even Lyndon Johnson understood it.
So in our Model America, you need to have a law that says "yeah we know that everyone is the same, but because a degree from [Prestigious University] has a ripple effect that affects society as a whole, we want to make sure that graduating classes have at least the CHANCE of reflecting the cultural diversity of the nation as a whole, so we need to have a law that prevents Bad Actors in Admissions from just saying 'Oh, we already let in all the white people in line, wink wink wink, sorry, maybe next year'"
That's the reason you might still want quotas. And given the distribution of test scores because everyone ISN'T identical frictionless spheres, you might want to add a weight to minority test scores to float them overall, so they get in.
And yeah, that might not seem fair if you're in the majority; or if you're in the minority whose test scores are highest, but there's a clear and self-evident purpose to those kinds of weightings. Life's not fair, but it should be equitable, overall.