In the end, it boils down to whether firms in a given industry have increasing or decreasing returns to scale. For example, there are no hairdresser empires as

In the end, it boils down to whether firms in a given industry have increasing or decreasing returns to scale. For example, there are no hairdresser empires as this tends to be a profession that can already be operated efficiently at a small scale. Other sectors, especially those with high fixed costs, tend to produce monopolies.