Currently buying health insurance is not buying access to a network of doctors and hospitals. It is buying limitation to a set group of doctors and hospitals to obtain coverage.
Medicare and Tricare work across all 50 states by providing access to ANY doctor or hospital that accepts their coverage - you are not limited.
But there is only one provider a doctor has to negotiate with in those cases, the US government. It would be much more complicated for doctors to negotiate and set up billing systems with perhaps dozens or even hundredz of insurance companies.
Because the government has told the doctors what they will be paid for their services. The governments risks are controlled because they have mandated a network and what that network can charge. Private companies have to go out and do that on their own. If a company is not currently doing business in my state and as a result has no network of providers set up then there is zero incentive for them to sell me a policy, regardless of how much I might want to buy from them.