Benefits only work if there's a large population where the risks can be averaged out. If you let people pick and choose whether they want insurance, people with known health problems will overwhelmingly want insurance, and the price will be driven up. Really this shouldn't be dealt with at the employer level but by the government - there's a net public good to everyone having guaranteed healthcare, because preventative care avoids larger emergency expenses and allows for more efficient allocation of resources. But instead you get this hyper-individualism where people want to opt into the social contract piecemeal at the time it benefits them, and opt out when it doesn't.