“Very few” isn’t the same as zero, never ever, under any circumstances. With Medicare reimbursement…and a good many commercial insurances, come to that…as low as it is, providers and facilities are becoming more reluctant to take a chance on not getting paid. I’m sure you can understand that. From your account of your time in the work force, I doubt you would’ve been too thrilled to find that a few hours of your working week weren’t being compensated without warning. Your healthcare providers aren’t going to be much different.
My husband had spent most of his career as academic faculty with a prenegotiated salary and never needed to think about denials or whether patients had referrals or needed pre-auth. Someone else had that burden. Since Covid, he decided not to sign another extended contract so opted for a flexible per diem arrangement. Still a prenegotiated salary independent of individual fees but, for some reason, he gets notification of any claim denial. It’s still Someone Else’s job to sort it out…but it’s focused his lenses a bit on just how close to not actually generating his paycheck he is with a large Medicare demographic (a feature of his specialty)…even when procedures are fully reimbursed.