Think waaaay back, to the Keynesian Cross model of the traditional first-year textbook:
1. Desired expenditure Yd is an increasing function of income (aka production) Y. So Yd = a + bY where a > 0 and 0 < b < 1
2. In equilibrium, Yd = Y
What is the process that brings the economy to the equilibrium of this model? The traditional textbook story talks about undesired inventory investment. If Yd < Y, then firms will be accumulating inventories of unsold goods faster than they desire, so will eventually cut production.
Now suppose that all goods are services, like haircuts, and it is physically impossible to store unsold haircuts. That traditional story cannot work. We cannot even imagine a state of affairs in which Yd < Y. What are firms doing? Dragging people in off the street and forcing them to buy a haircut?
How are we supposed to teach this stuff in a modern service economy?