Because many of the new cars, are new generations/evolutions of previous models.
Camaro, for example, has had 4 distinct versions. Each of these is referred to a generation. It makes it easier to define the body style/car. For example, instead of saying my 1982-1992 era Camaro, it's just a Third Generation.
The newest, makes Generation 5.
These terms have been around for ages, it's nothing new. Look at Corvette. A mid 70's is called a C3. The first set, was C1. And that was in the 50s. Same idea.