Because you are looking at it through the lens of total student makeup of certain top universities.
If Whites need 100 points higher on SAT than some others, then Whites are being discriminated against. That is, if you remove only the discrimination against Whites then they do better in the current system. If Asians need 200 points higher on SAT and you remove all discrimination then Whites do worse at some top universities than currently. So Whites would be both being discriminated against and for at those certain universities.
If you look at the overall college system, there's something like 35% of undergrads that are Black/Hispanic and 5% for Asian so in the overall system Whites net benefit more from no discrimination than Asians do.
So you've picked a particular point of view between principled and subjective, global and local, to conclude Whites aren't being discriminated against. Maybe it's worth exploring why.