Why are there only a small number of businesses in the oligopoly industry, such as 2-3 car makers in the U.S. or three large cereal makers that control 80-90% of the market share?

In an oligopolistic market, there are typically two or more firms whose actions or decisions have some impact on the other producers. There are usually a few businesses that make up a large part or even the whole market. The reason for this is that there are high barriers to enter the industry. The most commonly seen barriers are economies of scale, brand loyalty, licences and other legal conditions, access to resources, and very developed and complex technology. All of these barriers make it difficult or even impossible to enter the market by new firms. Even though the barriers to entry are lower in oligopoly than in a monopoly, they are still high enough to maintain the small number of producers in the market.