US health care is screwy, because one of the equitable things it tries to create is equal price regardless of age/sickness.

As a consequence, insurance companies generally try to sign up mixed-pool, younger, healthier (read: employee plans) over older, sicker, opt-in insurance (read: ACA).

There are ways they can and can't legally do that, but it's always the goal. In the same way that retail order flow is targeted by HFTs.