Because then you get into the problem of defining a coupe from a sedan. Traditionally, coupes have had two doors and a separate trunk, and sedans four doors and a separate trunk (i.e. no hatch lid integrated with the rear window), But that all began to change with the 3-door Saturn S-series coupes with the half-rear door, Mazda RX-8 sports-coupes and Saturn Ion coupes with the two half-rear-doors, and the Mercedes CLS and VW CC sedans with the four full-doors and coupe-like rooflines. So where does one stop and the other begin?
It is way more than that, and again it all has to with customer/consumer ignorance and business profit. The main difference between a normal car (sedan) and a sports car (mostly coupe) is in dynamics. A sports car, to fulfill its purpose, has to be faster than a normal car, so to achieve this you would technically focus on suspension geometry (double wishbone), center of gravity (dry sump, low roof height thus little head room), weight distribution (mid-engine, transaxle if front engine to get rear biased distribution), low overall weight (and thus loss of practicality like rear seats).
If a sports car has 2 seats compared to a normal car with 2 seat rows, so at least 4 seats, there is a niche in making a sports car with 4 seats, and so four doors. But now on the market, all German 4 door coupes are not pure sports cars with 4 seats and 4 doors, but just marketing products that work thanks to German badge prestige. I can think of only one true 4 seats sports car: the Ferrari FF/GTC4, but it has only two doors, 4 could have been possible. It has dry sump and low roof (low CoG), mid-front engine with transaxle (rear biased weight distribution), relatively light for its size (1.800 kg for 4.9 m).
While many cars on the market may seem interesting, they are mostly (because of platform and component sharing) nothing else than a slight variation of the same flavor, and consequently for someone as me claiming to be knowledgeable, uninteresting mediocrity.
The real change I hope to come will be with BEVs, where the differences between normal and sport are going to be totally blurred (already the case with suspensions settings that can with the flip of the switch be comfortable or hard, have little role or high articulation, think Toyota KDSS).
Conclusion: The job of defining is given to the marketing department, and that is an error, it must be given to the engineering department. I want products to be built by engineers, not marketeers. A 4 door coupes are not built by engineers, but one of them seems to be and is scheduled for 2018: Porsche Mission-E.
