The stockmarket and pyramid schemes are examples of, what I call, a 'Multi-Expansion Copy Machine' where the system grows by making larger copies of itself until it reaches a Limit, which can comprise many different factors, and then abruptly collapse.  Since they are groupable under analysis they have common properties and have an intersection of the set of their attributes.  (In English, mostly they are the same but the differences are important.)

To an economist stock markets are Good/pyramid schemes are Bad.  Thus, economists resist seeing the commonalities.