That just means you are locked into whatever hospital makes the deal that is most advantageous FOR THE INSURANCE COMPANY. That doesn’t mean it is the best deal for me, either financially or in terms of outcomes.
Hotel rooms at similar points on the price scale are pretty much interchangeable, and if you get a room at the Ramada versus the Days Inn, it’s not going to be particularly significant in the long run. Similarly, it’s no big deal if my auto insurer has rental-car arrangements with Hertz versus Enterprise versus Avis–they’re all relatively similar, and they all get the job done. I would never pick an auto insurer based on which car rental place they have a deal with (quite aside from the fact they can change their contractor mid-year).
Health care, though, is fundamentally different. Different hospitals have quite different specialties and areas of expertise; somebody upthread, for example, pointed out the difference that a stroke center can make in how much function you regain. In my town, one of the two major hospitals has an advanced stroke center, and the other one doesn’t. That doesn’t mean the other hospital is “low quality”; it just means that for that particular health problem, they don’t have quite as much expertise, equipment, staffing, etc., to achieve the best possible outcome.
Now it may make perfect sense for my insurer to shop around and determine that for THEM, Hospital B is the most advantageous, because on average their costs will be the lowest for a reasonable level of care. That does not imply that costs for any one health problem will be lower; they might be higher on some and lower on others. The insurer pays for the aggregate, but I pay for my own health problems. I might well end up paying more at Hospital B than at Hospital A, even though the arrangement is better for my insurer because they are paying for thousands of patients with myriad health problems.
Moreover, the cheaper outcome based on direct costs isn’t necessarily the cheaper outcome long-term. If I don’t recover as much function after a stroke, that might not be my insurer’s problem at all–for example, if I can’t return to work, my health insurer has no obligation to pay for long-term care or home health aides or other kinds of assistance. That’s a different policy with a different company. That’s Somebody Else’s Problem. As long as the care is good enough and cheap enough across all of the members of the group plan, whether it’s good enough and cheap enough for ME isn’t a consideration for my insurer.
In other words, your niece contributed exactly nothing to shopping around based on price.