State lines = fiefdoms and potential monopolies.
Car insurance doesn’t have that problem.
States should have rights, e.g., insurance does not have to cover abortions. If ins. companies want to sell in another state, they can set up an office there. Blue Cross is ubiquitous but can't pay the high costs any more easily than any other company, without astronomical premiums, and they are quite the fiefdom compared to smaller companies, indeed.