Probably because its at best meaningless, and at worst misleading.
For example, in the Camaro there are 3 different 6.2L engines used: the LS3, the L99, and the LSA. Sure, if you know your stuff you can figure out one a particular car has. But then again ... if you know your stuff, a badge isn't going to help. Now, consider the Impala. Its 3.9L engine was replaced with a 3.6L. If you only kinda know about cars you'd think this was a step backwards. But in reality, it was a big step forward.