So much this:
Moreover, an out of state insurer has to be able to develop and negotiate rates in a market to compete anyway. Which is hard to do if they don't have a very significant customer base in that market.
The barriers to entry are huge because of state laws, local laws, negotiating with networks of providers, etc.
And, by the way, when health insurers negotiate with providers, guess what, this is an instance where competition can actually make the cost of healthcare more expensive.
Let's say you have one provider network and one insurer. So eah side hires, say, two lawyers to negotiate, so four lawyers total. Customers end up paying for four lawyers.
Alternatively, let's say you have a dozen providers and a dozen insurers. Now you have 12*12=144 negotiations, each side of each negotiation consuming two lawyers puts you at 576 lawyers. Now, customers end up paying for 576 lawyers.
Walmart saves money through consolidation and bargaining power. Single payer makes use of those concepts to keep prices down.