USA employers started offering health insurance during World War 2 as a means of enticing women to come to work, as there weren’t enough men to fill the positions. And once that trend started, it simply never stopped.
It's true that they started offering health insurance during WWII, but it wasn't specifically to entice women into the workforce, rather it was to be competitive when Roosevelt created the Office of Economic Stabilization, which froze wages. Offering health insurance was a way to get around this.
https://www.nytimes.com/2017/09/05/upshot/the-real-reason-th...