You are wrong. The states control what products can be sold, and each imposes there own regulations. Some are very sparsely regulated, others are a nightmare of bureaucracy. It’s not just a matter of registering.
Perhaps I wasn't as clear as I should be. Sure registering with a state's insurance commission requires that the insurance company comply with regulatory requirements. But why would I want to buy a policy from a company that does not currently do business in Missouri? And why would they want to sell a policy to me?