Why do markets dominated by oligopolies result in high prices for the consumer?

In oligopoly, the market is controlled by a small group of entities which control all levels of production in the market. This make it extremely difficult for new competitor to arrive and compete with the existing group control the market.

When there is no competitors to worry about, the consumers have no other options beside buying the products that the group produced. This is why they can raise the product as they like without worrying about losing customers.