There are a lot of factors that go into exhaust manufacturing. A lot of times, a company producing an exhaust will start with the easiest method to produce both power as well as get a better sound. Keeping in mind that resonance and vibration are huge factors and have to be dealt with accordingly. Many times this cannot be done with a small amount of components.
Take my Ark Exhaust for my Optima for example. The full system from Ark has (1) Cat, (2) Resonators, (1) Helmholtz resonator, and (2) mufflers. It is hands down the best exhaust system out of the 6 or 7 that are on the market. It performs the best because the piping diameters are correct and it has almost no drone due to the (3) resonators. Without them, the car is nearly unbearable to drive in. Ark went through 12 prototypes before they put my exhaust into production. I know this first hand as I'm friends with the owner of the original vehicle and we have a very close relationship with ark, especially since we're only 30 minutes away.
While your stock exhaust might perform well, once you add the mufflers, it might increase the resonance in the stock piping adding drone and cabin noise. This would then have to be dealt with by adding resonators and such further up the line. It's kind of a trickle down effect but you can pretty much guarantee that if there were a viable axleback option that was a good balance between power and sound (with minimal drawbacks) that it would be done since they're less expensive to produce and a better price point to sell at. I know there's one from Jun BL I believe, but it's expensive and I've never heard it in person nor have most on these forums I believe.
Now, I'm not trying to sound like an Ark fanboy, it's just an example that I can personally relate and speak to.
