There is this old joke about how even if you are a one-in-a-million person, there are still 8000 of "you"s to choose from; how does one chose who to pick, if there aren't positions for all of them?
So how do you make it "fair"? Since of the number of seats are limited, so imagine if the cut-off for the last seat is for someone who is in the 90.3765335th percentile, look at how many significant digits that is! (Yes, that's how specific it can get, when population sizes grow large enough.)
Yes, you can argue about having a "bad day", but there are thousands of other people ALSO in the same percentile as you, and you all didn't have a "bad day", at some point it's about resource limitation.
There are two and a half billion Asians of different varieties, that means MILLIONS of people at each economic/social strata. You simply CAN'T provide the same resource to all of them, some of them will have to vie for a lower-ranked resource.
If there are limited number of seats, and your economy cannot afford to fund more seats, then a standardized test is the only fair way, at least it allows you to have SOME sort of control at your luck.
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That same thinking applies to Asians who move to the US. It's patently unfair, from their PoV, to give weightage to things they CAN'T control for (race/wealth etc.) and not give weightage to a thing they CAN control for; a standardized test.