Market dictates what sells. Car companies consider a lot in making a car. They are trying to maximize sales and that means juggling several factors.
There are several reasons you don't see boat tails. One is its much more expensive to ship longer cars because you can't fit as many on a truck or a train car. Another is if its too far out looking no one will buy it as it was the case in the 70 Dodge Charger Daytona . Then you have insurance companies getting involved telling the car makers that they will charge more to insure cars with tails that stick out because people are more likely to bump into things with it.
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