Most likely because there is absolutely no benefit to doing so. 

Health insurance rates vary between states, but as a general rule, health insurance is less expensive for healthy individuals than groups (because of underwriting).   Since the indy firm wouldn't be paying any of the premium, the healthy individuals will buy their coverage on their own.  The unhealthy ones would buy the group coverage.  The end result would be adverse selection which ultimately will cause the group to be very expensive. 

Who says they don’t? Maybe you just haven’t checked them all.