What are some reasons that popular culture defines the 1920's as a time of great economic expansion?

In many ways, the 1920s was a time of economic expansion, but the fundamentals of the economy were weak. Popular culture has perhaps seized on the appearance of greater prosperity in the decade, in part popularized through advertising. Modern advertising campaigns took off in the 1920s to promote a range of new consumer goods that were available, including less expensive cars, washing machines, vacuum cleaners, and other goods. In a more competitive marketplace, producers turned to advertising to promote their goods. Ads tried to create a demand for products that perhaps were not needed before, such as new types of beauty products. 

In addition, many consumers purchased goods on credit, meaning that they financed the purchase of products that they could not otherwise afford. Therefore, people looked more prosperous than they were, and this sense of affluence, celebrated during this decade, became its hallmark in popular culture.

However, underneath the surface prosperity, the American economy was heading towards the stock market crash in October of 1929. One of the many reasons behind the crash was that people's income was not increasing in line with the rapid increase in the production of new goods. Over time, people could simply not afford all the new goods being produced, though this reality does not always figure into the portrayals of the decade in popular culture.