It's exactly the other way around.
You'd be right if YC were perfect at assessing founders every time, but of course they're not. Applying repeatedly gives you a chance to prove that they got you wrong the first time (and the second, and the third, and so on, if need be). Not only does this work, it's the majority case (I just double checked this). Most founders that YC funds are repeat applicants.
Applying repeatedly demonstrates persistence, which is one of the qualities YC looks for. Better still, if you can show continued progress since your previous application, that demonstrates resourcefulness, another core quality. Repeated applications can move the needle on what sort of founder YC thinks you are. Especially if you were a borderline call the last time—keeping going, and applying again, is a way to get on the other side of the line.