That's not entirely true. The middle class used to be known as the bourgeoisie, and before that they were traders, merchants, and sometimes small-scale artisans, who were below the local aristocracy, but above the peasants, serfs, and cannon fodder.

The US has a peculiar definition which includes workers who would normally be considered working class. The true middle classes are usually professionals with advanced specialisms that require above-average intellect and education, and typically include some element of management and administration. They have some political leverage, but typically less than they think.

This version of the middle classes emerged as the administrative, technical, scientific, medical, and legal support for company owners and speculators during the Industrial Revolution. This is still recognisable today. (It was a huge problem during the Maoist revolution, and Mao went out of his way to destroy and "reeducate" this class - because they were capable of resisting him.)

The upper classes rely on inherited wealth and power, make their money from property, land, share ownership and other forms of rent seeking, have direct personal connections to networks that shape policy and power, and don't need to work unless they choose to.