You’re forgetting economies of scale. Let’s take phone plans. A few giant companies have infrastructure (cell towers) built across the country. Coverage is extremely important - a phone plan with coverage in a small area isn’t anything anyone will want. How is a third party supposed to compete? They’d need enough money to set up nation-wide infrastructure, contracts with phone manufacturers to make sure phones are compatable, and they need to do all that before they even sell anything. Even if you try to compete, how do you make your prices competitive after spending that huge amount of money?