The biggest barrier to competition is the billions of dollars required to start up a new car company. The last successful new American car company was Chrysler in 1928. That’s not the reason.
You have plenty of new car companies building cars in the US. Foreign car makers with assembly plants, parts makers, Tesla, etc. Its no longer the vertically integrated industries of the 1950’s. And there are imports from new producers around the world. Easier regulations mean lower costs of entry for new models from domestic and foreign makers.