I don't think the size of the pool is that relevant. The pool from the perspective of insurance company is all insured people, so it doesn't matter whether it's a company of 5 or 5 individuals joining, it's still a huge pool.
Adverse selection is the big reason. An individual signing up for insurance can be used as a signal that means "individual is sick" or "individual is likely to need insurance soon". If a company policy covers all employees (or all employees above a certain level), the signal is "person works" which is orthogonal to "person will need insurance" (in fact, it's probably slightly anti-correlated, a.k.a. "person is fit enough to work").
Adverse selection is the big reason. An individual signing up for insurance can be used as a signal that means "individual is sick" or "individual is likely to need insurance soon". If a company policy covers all employees (or all employees above a certain level), the signal is "person works" which is orthogonal to "person will need insurance" (in fact, it's probably slightly anti-correlated, a.k.a. "person is fit enough to work").