What made clic american muscle cars so strong?

i was playing Need For Speed Carbon a while back and i see all the muscle cars are the clic american cars that every white or black (because they are americans from the beginning) dreaming to get a Shelby 300GT or Plymouth Hemi Cuda. why were they so powerful back in the day but today the same cars which are being produced are so weak. take for example the the Impala SS. people were so crazy back in the 70s and 80s becuz they were fast with a lot of power and RWD and comparable to probably todays Nissan Skyline GT-R or Mitsubishi Lancer Evolution. but today Chevy turned it into a Honda Accord. why?