With all the extra coupling hardware and surface area required, the reason is performance and weight. Modern cars are all about material minimization. It's easi

With all the extra coupling hardware and surface area required, the reason is performance and weight. Modern cars are all about material minimization. It's easier to add a bump to the cabin and put a motor in that volume than it is to make another mini car. I think it would only make financial sense if it were like swappable batteries: you rent it all.