When economists think about market structures, they include market structures like pure monopoly and pure competition that are very rare in the real world. why do economists spend time thinking about conditions that almost never exist?

As explained below.

Explanation:

Asan economist the market is characterized by the interplay of 4 major fields like the Oligopoly, Monopoly, Perfect competition, Monopolistic competition. Thus these structures were given by Adam Smith and his fellow Karl Marx. As an economist, he emphasized the laissez-faire model that is hard to find operating in a market as of the existence in 20 and the 21st century. The presence of the monopolistic competition which is a type of imperfect competition is that many producers sell or services that are different nature without carrying for the prices. The oligopoly structure is either run by duopoly or monopoly or by the Ologospony which represents the market by many sellers but the demands of the few buyers. The presence of perfect competition creates certain barriers ronery of the producers and consumers. However, these all tend to differ from the natural monopoly of one provider of a group of series meeting the one market.  Hence certain market structures are related to the pecking and competitiveness in the market domain thus economists have o study them is greater depths.