Why do we include only the variable cost of sales when estimating the average investment in accounts

Question:

Why do we include only the variable cost of sales when estimating the average investment in accounts receivable? Why do we apply an to this investment to estimate its cost?
Opportunity Cost
Opportunity cost is the profit lost when one alternative is selected over another. The Opportunity Cost refers to the expected returns from the second best alternative use of resources that are foregone due to the scarcity of resources such as land,...