That is a major factor, but there is a bigger one: the product that an insurance company will agree to sell you and the product they will agree to sell your emp

That is a major factor, but there is a bigger one: the product that an insurance company will agree to sell you and the product they will agree to sell your employer are not materially the same product. The actuarial assumption is that substantially all white collar employees will get group coverage and therefore any company's 25, 100, or 500 pulls out of the pool of white collar employee health risks will be actuarially consistent with random selections from the underlying distribution. This is not true of individual insurance purchasers, because the individual purchasers can have more signal on their health risks than their insurance companies. They can, for example, purchase insurance when they know they're likely going to require treatment and decline to purchase (or shift to different coverage options) when they're likely to be healthy. This is called "adverse selection" and it is the tradeoff in individual insurance. For a while, the US's policy was that insurance companies could underwrite individual health insurance policies in a fashion roughly similar to underwriting business liability policies, which could be caricatured as "We'll give a policy to anyone with a pulse but if you've so much as glanced in the general direction of an expensive outcome at any point, our answer is Hell No, just like substantially every other insurance company's will be." If you let insurance companies do underwriting, health insurance is (relatively) cheap and lots of people can't buy it on the individual market at any price. If you require "guaranteed issue", health insurance is only (relatively) cheap if everyone buys it. The US has been wrestling this tradeoff with Obamacare and subsequent proposals. There are no simple solutions to it that make everyone happy.