Health insurance will often initially deny coverage, either in the pre-approval or when you get the bill, but if you firmly follow up and hold their feet to the policy they will reverse that.
Even if you're uninsured, healthcare providers will usually settle medical debt for about 10c on the dollar if you threaten bankruptcy.
Home/auto insurance will often give you a black hole for claims; the adjuster won't get back to you for months at a time. But if you keep calling them enough to be annoying, they will usually eventually pay out.
Contractors will often fail to fix shoddy workmanship until you threaten to report them to the local licensing board.
Cops will often ticket you, but a surprising number of tickets get reversed if you contest them in court, oftentimes because the cop doesn't even show up. One of my friend's parents in high school vowed never to take a speeding ticket lying down; he was 5-0 on contesting them.
The IRS will come at you with the interpretation of tax laws that is most favorable to them given the data they don't have available, particularly if you are high-income. But if you actually know the laws and can show hard data (statements/receipts), you can usually get the bill they claim you owe reduced by 80%+ or even to $0.
Colleges will often increase their financial aid packages by thousands of dollars if you have a competing acceptance and are like "Well, I would love to attend your school, but financially it's hard for me to justify paying $X more than this other school I got into."
Companies will often increase their compensation offers significantly, sometimes by hundreds of thousands of dollars, if you turn down their offer.
It's all negotiation. They put forth an initial low-ball offer because most people accept it. If you make it clear that you are one of the folks who will make a stink about it, they'll often increase their offer significantly to get you to go away.
If 90% of denials that are challenged end up being reversed, and the delay in delivering the care that the insurer contractually agreed to deliver is deleterious to the patient's overall health and leads to prolonged suffering, why is this not a cause of action? Until there is some sort of downside introduced, the "deny til they die" health insurance model is going to continue to thrive at the expense of people not getting the care they paid for and are entitled to.
This is representative of a general problem with the legal system today. It's too expensive, and operates too slowly. When the economics of cheating people were that you might be able to cheat 10 people before you pick on the wrong guy who has a lawyer that puts you out of business, you have an incentive to follow through on your contracts and do what you say you'll do. When the economics became that you can cheat 10 million people before you pick on the wrong guy who sues you, it becomes rational to take the money from those 10M people and use it to outspend the guy who sues you, or at least just pay him off so he goes away. And so our western notions of democracy and contract law get subsumed into a feudal system where only big corporations get protection of law, and you must join the local protection racket to slot into that system.