> Kaiser Permanente operates 39 hospitals and more than 700 medical offices, with over 300,000 personnel, including more than 87,000 physicians and nurses.

https://en.wikipedia.org/wiki/Kaiser_Permanente

The second largest also owns Optum with 60k doctors:

> Optum Care is a family of 60,000 doctors in 2,000 locations nationwide. We work together to help 20+ million people live healthier lives.

https://www.optum.com/en/about-us/optum-care.html

So I'd ask you back, how many insurance companies aren't under the same parent org as hospitals? The 2 largest are, wouldn't surprise me if most are.