Getting it replaced is different then having one installed. Most likely your power steering pump seized up and you had that replaced (most common PS problem). Essentially a power steering system is a hydraulic system. So you need the pump, reservoir, lines, filters and actuators to make it all work. So to have one installed in a car that doesn't have one is a nightmare. Trying to make it all fit without interfering with the current gear, how you are going to power it etc etc. Then if you do have problems it will cost a [Poor language removed]-load to fix as there will be no warranty to fall back on.
Too much of a pain in the ass, too expensive (parts AND installation) and it will more than likely break. And driving a car with a broken PS system is WORSE than driving a car with no PS (I did it after seizing my PS pump in my WRX doing donuts in the the snow, had to wait a week for the new pump to come in and it was a BITCH to drive)