Why Is Individual Health Insurance So Expensive?

I just can’t quite wrap my head around why individual health insurance is so expensive. I do understand the basic idea that the more people there are in an insured group, the better their collective risk is spread out among them. So, employer health insurance is cheaper per person than Self Employment Health Insurance, which often only considers the risk of that one person who possesses it. Yet, at the end of the day, isn’t the insurance company simply serving one big group of people, of which many individuals are a part? One company doesn’t work for only one person. Why, then, does it matter whether or not that one person is in a sub-group as long as she’s part of the whole group?