Because it's designed to be cheap, rather than efficient.
I think what you meant to say is designed to be cheap
to operate. Even an engine with high manufacturing cost can be very economical to operate if it is used many times and requires limited servicing.
You bring up a very interesting point regarding the trade off between cost and efficiency. Long before Elon Musk and SpaceX came along, there was a very bright entrepreneur named Andy Beal.
Beal Aerospace's goal was building a heavy launch vehicle that would drastically reduce the $/lb cost of commercial launches. But rather than pursuing a reuseable design, his approach was to make the vehicle as simple and inexpensive to produce as possible. Of course the trade off was reduced performance.
The
BA-2 heavy launcher used a single massive peroxide/kerosene engine for each of the first two stages. The engines were pressure fed from the propellant tanks and did not require turbo pumps. The composite propellant tanks were pressurized using helium. The engines used un-cooled (ablative) nozzles. Beal actually made quite a bit of progress with his limited resources. To give you an idea of the size of the engines, here's an image of the second stage engine being test fired:
