That's a system that Oxford explicitly rejects. They even give a test for science candidates, and then don't really care about the results (they're just there to check that you're not completely ignorant).
Oxford selects for potential, not for test results. How do they judge that potential? Well, they have developed detectors which are really advanced, carefully calibrated, extremely sensitive and incredibly difficult to game.
Those detectors come in the form of "professors" who have been teaching for 20+ years and know how to judge a student's potential from a 20 minute interview.
You need to take into account that test scores don't tell you enough. Currently roughly 8% get the top grade (A star) at A level, per subject [1]. I can't find a number for what %age of students achieve all-A star grades, but I expect it's still greater than the number who apply to Oxford or Cambridge.
It's the job of the interview to distinguish between those who have the potential to go beyond what's needed for A levels. For the most part, if someone has had to work extremely hard to achieve the top A-level grades, they're going to struggle at Oxbridge where the level is that much higher.
I'm not saying the current interview process is perfect, but test scores alone don't tell you enough to distinguish the best students.
The only exception are the winners of nationwide competitions, who can enter any university they choose.