Expense. Much of engine design is getting rid of heat, which is much harder in the wind shadow of the car. It also makes it harder to have a good weight balan

Expense. Much of engine design is getting rid of heat, which is much harder in the wind shadow of the car. It also makes it harder to have a good weight balance, and you end up adding significant engineering to keep the rear heavy car from having undesirable handling characteristics ... which is something the Porsche 911 has been battling for decades.