Why is 'revenue' the proper denominator, and not 'earnings' ? Sure, if revenue hypothetically vanished, cash could cover it, but a lot of revenue is tied to ex

Why is 'revenue' the proper denominator, and not 'earnings' ? Sure, if revenue hypothetically vanished, cash could cover it, but a lot of revenue is tied to expenses ("cost of goods sold" in the retail world), so if revenue dropped, expenses would as well.