Why? And how else would you describe the economic strata that exist in our economy and every other economy in the world?

People are people. We have a classless society in the U.S.. It would be far more correct to refer to differences in income, rather than using the word "class".

Class is an indicator of power, which in our capitalistic society, is largely determined by income or wealth. Using the terms "lower, middle, upper class" mostly does speak of differences in income[1], though the upper class is mostly determined by wealth and a return on capital instead of labor.

[1] https://en.wikipedia.org/wiki/Social_class_in_the_United_Sta...