It's analogous why you can't just pay for your hotel bed and shower, but not the coffee pot, the valet, not the hotel, nor the food court. People are not willing to pay the TRUE cost of something, so costs are aggregated and distributed over prices in a way to achieve the targeted revenue the business desires.
People have this false belief that if I only want 1/10th of the product, if I were to pay ala-carte I would only need to pay 1/10th the current price. That's fantasy and not real - most businesses are not linear in their costs that scale all the way down to a single unit. Or if they tried, no one would be willing to pay that type of price.