[AnthonyMouse]

That's assuming you need the controller to do the exact same thing in the exact same way as the OEM one. But in many cases people are replacing it because they want it to do something different anyway.

And these functions are in many cases quite simple. You replace the controller for the door locks. It operates a solenoid that locks and unlocks the doors. Maybe they no longer function by calling the call center to have them unlock your car, but maybe you don't want that anyway. They still function if you press a button on your key fob.