When we look at the ease to enter the different market structure, there is no doubt that 'monopoly' is the hardest. Why?

When we look at the ease to enter the different market structure, there is no doubt that 'monopoly' is the hardest. Why? -- There is only ONE firm that has established 'economies of scale' with the production of their good or service. If another competitor came close to competing with them, they would simply lower their price (even in the long term) and still remain profitable. is this why the Department of Justice attempts to eliminate monopolies from the United States?