Documented how? In the immediate aftermath of the attack, UnitedHealth explicitly stated it wouldn’t change its policies:

UnitedHealth Group CEO Andrew Witty defended the company’s claim denial practices in an internal video which was leaked after the fatal shooting of Thompson. Recorded on December 5, the day after Thompson’s death, Witty emphasized the company’s role in ensuring “safe and appropriate” care and that the insurance giant will continue to prevent "unnecessary care”.

It also demonstrates that these companies don’t HAVE to reject all these claims, they’re doing it because they want to.

Sure, and poverty can be solved if rich people would just give some of their money to poor people. But we know this isn’t a solution that actually works if you want to fix poverty, you need government legislation.

Even if there was some health care insurance company that suddenly saw the light and started approving all the care people needed, it would just go bankrupt in favour of a more unscrupulous competitor - as long as the system and legislators allow it.