Why do goverments often regulate buisness in a capitalist society

To prevent monopolies and oligopolies in economies.

Explanation:

Monopolies are organizations that have almost entire control of markets. If I ran a monopolistic oil company in the US, I could in theory control the cost of oil by raising it higher/lower than any competitor and charge lots of households many more had there been competition.

Oligopolies are organizations that work together to cut out competition, with the exception of the corporate ally. If my buddy and I ran an oligopoly on oil, we would have control of the entire market together and control the costs of oil, in theory.

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