Most monopoly or duopoly situations persist due to government capture i.e. burdensome regulations on competitors and favoritism through lobbying efforts that startups likely cannot affort.
Discussion
GM, exactly
As Mises says:
„The great monopoly problem mankind has to face today is not an outgrowth of the operation of the market economy. It is a product of purposive action on the part of governments. It is not one of the evils inherent in capitalism as the demagogues trumpet. It is, on the contrary, the fruit of policies hostile to capitalism and intent upon sabotaging and destroying its operation.“
He explains it in long form in chapter XVI of Human Action. It’s free to read: