1) If you don't get certain very expensive medical care, you DIE. So if you can't afford it you, you are likely to consider stealing someone else's medical insurance. Death makes people consider doing things they wouldn't otherwise do.
2) Many patients with health issues have a lot more important things to think about than finances. Or worse, the patient might be dead, so they can't complain against the charges.
3) Many providers actively avoid talking about finances. Not only do they know about point 2 above, but they also fear that if they talk too much about the finances, you will realize how badly they are screwing you over and might actually look for reasonably prices services.
So when you steal the Identity of a patient with serious medical problems, there is an inbuilt set of honest people willing to buy the information you stole, the victims may not be in the best shape to investigate, and no one else wants to look to deeply into it.