Easy rpm.
Same reason Chevy will not do that, is the same reason the T-Bird turned out to be a luxo-boat.
Ford is afraid of hurting Mustang sales and Chevy will not tread into the Corvettes sales area with another car.
Even Chrysler has made an error or two, but is leagues ahead of the others. The Viper works and makes money for them as it does exactly what it looks like it does. The Prowler, on the other hand, looked fast, but was a slug and sales flopped.
The PT looks like a little truck/panel van, so no one expects it to keep up with a Viper.
I don't know who the American card makers use for focus groups, but they need to fire them.