Developing that sort of expertise requires getting clear and—ideally—timely feedback on the quality of your decisions. Do parole judges get that?
I'm sure they get feedback on whether their decisions are consistent with what other judges would have decided, but that's qualitatively different from feedback on whether those decisions were fair or right. If anything, that is the kind of feedback that would propagate biases in the system! You would end up becoming an expert on making consistent, defensible decisions, even if those decisions were consistently and defensibly bad.
I'm sure they get feedback on whether their decisions are consistent with what other judges would have decided, but that's qualitatively different from feedback on whether those decisions were fair or right. If anything, that is the kind of feedback that would propagate biases in the system! You would end up becoming an expert on making consistent, defensible decisions, even if those decisions were consistently and defensibly bad.