I was wondering the same thing not long ago, and I was surprised to find that health insurance for small businesses can be quite affordable. In fact, sources like this Taylor Benefits article say that the cost of group health insurance for small businesses has been trending downwards over the last few years. The cost can be as low as $270 per employee in some areas of the country, depending on the type of coverage you spring for.
Employee healthcare also costs a lot less than it costs the average American to get the same level of coverage. There are many reasons for that, including legal incentives and differences in how companies approach selling insurance to large groups. On top of that, insurance companies are also willing to get smaller profit margins when looking at the possibility of landing several new clients at once, especially given that companies tend to stick with the same insurance provider as they expand and hire more people.