People can't opt-out if they're getting a bad deal.
Risk outliers get turned down as bad business decisions, or the drugs they need to stay alive are impossible to afford because of "low" demand.
It's left up to employers to choose the product that's best for their employees (and ensure that market competition functions), but that assumes they actually care about their employees' well-being, which most don't.
Denying care to the unemployed is a strong force deepening the poverty cycle, preventing people from working their way back up to being productive members of society.
Healthcare doesn't even really fit the model of insurance, because it isn't just a guard against unlikely catastrophe, it's an ongoing, chronic, inevitable cost of being a human being.