Its not just a wire Mouse, its like a dongle that changes the information that your I-phone sends out and translates it into a language that your car can understand. I dont know of an aftermarket solution but they may well be available.
They are not included with the car becausse not everyone has an I-Phone, it would be a huge waste of money on Fiat groups behalf (dont forget its the same for the Fiats and Alfa's) if they supplied the unit with every vehicle.
It is as much Apples fault that the I-phone isn't completely compatible with the car but at least Fiat group have put a solution in place.