My understanding is that this is only really true for straightforward things like, say, a therapist. If they only have a couple of codes that they bill and they accept a limited number of insurance providers, then they can probably tell you what you'll pay (although I believe there are still a lot of edge cases).
However, if it's something like a surgery at a major health system, then it's way more complicated. The health system can't be as selective about what insurance they take, so they're dealing with medicare, medicaid, plans sold on the individual/small business market, and employer-sponsored plans. So way more than a few providers and a few types of plans. I checked the stats for my state and just the individual/small business market is 12 providers and 250+ plans. Medicare Advantage is at least 14 providers. A major hospital system probably accepts thousands, if not tens of thousands of different types of plans. Then you have to consider that the anesthesiologist, the surgeon, and the facility are all separate providers who may not all take the same insurance.