The answer is that the manufacturers want you to! That’s how they make their money.
The lifetime cost of all the cartridges you buy is typically many times the original cost of the printer, and manufacturers try to ensure you keep on buying them.
It’s a classic marketing technique called "Razors and Razorblades!"
They sell the machine cheaply and the consumables for a small fortune!
So save money and try a TonerTopUp now!