We know how to build cars. We are stifled by emissions controls and part prices. The fact is, the reason you get a 6.3L v8 with a sad 425bhp is because you can buy the car, get a better intake, exhaust, bigger injectors and a tune and make 550 at the wheels.
If that's not enough for you, you can add a supercharger or turbo system for next to nothing and make well over 1,000. If they delivered all the potential the cars are capable of, not only would the cars be very difficult to control, very difficult to make safe and emissions-worthy, but the companies would be forced to list a price over $100,000. Mostly from taxes. No one would buy them then.
The point is, you can buy these cars for $40k, invest maybe $10k into them, and embarass owners of $500,000 cars driving down the highway and on the drag strip(let's be honest, that's what they were made for anyways)
I must agree, though. These companies are really only making pony cars nowadays(mustang, camaro, challenger). and the corvette/viper/ford GT are really just mid-range supercars. Pontiac had the G8 for a while, and Cadillac has the CTS-V.. but that's as close as you get, really. I kind of hope they don't bring back the old names though, like chevelle, they'll probably just ruin it.