Is a basic economics technicality.
If T10 were to design a model in which they sold a car individually (which I think would be something like 1.3 USD as the packs have one free car, I think for FM games though I'm not sure for FM5) it would be less profitable because if they were to sell them individually they wouldn't get the same revenue as they would either invest far more in licensing cars that people would buy, or loose more for not selling cars within the the car batches they dispatch.
To circumvent such an issue they set up a model in which they release a number of cars to cover different demographics (i.e. a classic car for enthusiast, a sports variant for people who single mindedly like one car/brand, etc...), which at the end of the day works better for T10 as they get a better revenue for the sold packs and the incentives they put on them.
I did ponder the issue because my main gripe with DLC models is that they shouldn't have a minimal cap of over 5 USD (this of course is subjected to the type of game is design for), I'm not sure about racing games but the problem for them is getting the licenses in the first place. In FM4 it made more sense because the car variety was wider and they had to acquire more licences, therefore it was an additional investment aside the budget assigned to produce the model.
IMO it depends more on the car pack, as each car has a "technical" value (production value vs licensing value) which means that they are not under a baseline of 1 USD, some of them would technically turn a profit when being priced at 0.80 USD while others would turn a profit for 1.30 USD (this taking into account that licensing something like a Mazda doesn't cost the same as licensing a Ferrari).
I think is fair as it is, but the problem would be that most of those cars are from older games, and therefore they potentially still holds the rights and licenses from previous instalments (as the Reventon for example), making a car "technically" valued at 1.30 become an asset valued at 0.80, making an hypothetical +0.4 gain from what it would be a more expensive car to produce.
IMO is a rip-off, but not in the sense of giving a single iconic car per pack to make people buy the whole pack, but by using an existing car in the franchise as an included asset in the pack (yes, FM games had great car listing but stuff like the Lambo Veneno, or a few other than in older instalments would have been produced are not produced to gain that 0.40 USD from the refurbishment of that specific asset).
Not supporting
UkHardcore23's view of the current DLC model, but how it seems to be degrading in order to turn more profit by making minor tweaks in the existing model.