Health insurance company margins are meaningless. All the large insurers are actually "pay-viders", meaning they own huge (and growing) chunks of the physicians (providers) they pay out.
Go read STAT's Colossus investigative series on the myriad ways UnitedHealth Group abuses this structure. All the other pay-viders do the same.
Pay-viders can "generate x% profit" for any value of x. They control all the relevant levers, which also by coincidence eliminates any incentive in the entire system to reduce costs.
They are required by law to only make a certain value of x, and so, surprise surprise, they all make x!
Complexity.
There are too many parties. There are too many bespoke party-party contract. There are too many new middle-parties being created, with their own contracts. There are too many systems. There's too much tech debt.
That's a reason improvements to the US healthcare system have generally been via HHS/CMS imposing standardization mandates ('or no Medicare payments').
But providers, facilities, and insurers (most operating on low margins) can't be collectively more meta-efficient without simplifying the system.
Single payer / Medicare for all is a critical start not because of coverage, but because of simplicity.
There are so many intractable problems with the current system simply by virtue that changing anything about them requires 100s of parties to update 10,000s of party-party contracts and interfaces.
That's the biggest source of waste in the current system: constant duplicative re-engineering.
Where do you think that money comes from?
You could just delete united healthcare and save bucketloads of money. They don’t actually do anything anyone wants, other than extract money as a middleman.