You are right in principle but the outcome of the interview is a combination of two factors: the baseline of real skill and the "noise" of your preparedness, mo

You are right in principle but the outcome of the interview is a combination of two factors: the baseline of real skill and the "noise" of your preparedness, mood on the particular day, judgement of the interviewer and so on. You can't change that, therefore it makes sense to put effort into improving the part of the noise that you can control. And also: a good preparation actually increases your real skill baseline.