That's because of the correct # of cylinders and the displacement.
Your 3.8 liter transversely mounted, crappy style'd factory exhaust placement won't ever sound like that.
You want something that isn't chambered...think Borla, Corsa, SLP, Dynomax...
The factory catback performance wise won't net you anything. If you're just looking for sound...buy a universal muffer and whatever tips you want to run, hack out the resonator and replace it with a long glasspack..think 20 inches or longer (30" max)...and if you want hack out the cat if you don't have emissions...or if you do...just toss a universal Magniaflow HFC in there. That'll change up the tone as well.
