Almost all healthcare claims are governed by ERISA & federal law. Despite allowing some regulation under things like the Savings Clause, a majority of federal law pre-empts state law. Things like auditing insurance companies over denied healthcare claims would almost certainly be pre-empted if they tried to take action. Case law on this subject is very broad.
You can even do some quick cursory searches to see how much this topic has been litigated & how insurance companies get away with so much because of it. They had plenty of times when Democrats controlled both branches to fix this & they chose not to do it. Good luck trying to get Congress to fix this now.