Because the client's goal is often cash, while the transaction itself involves buying and selling a commodity.
Steps
If you lose sight of the steps, you'll only see "I received a sum and will pay back with an increase," and you won't understand where the real contract lies.
That's why, specifically in the case of tawarruq (commodity murabaha), ask for a detailed explanation of the commodity's journey, ownership, and sale, not just the installment schedule.