Because people enjoy spending money on, and working on their cars, that’s why! The stock system is very good, yes. But some of the aftermarket systems are better. Even if only slightly. I did mine for as much the enjoyment of modding my car as I did the pursuit of something better. Having said that, the ATAK is better than stock. But, it’s exhaust so the sound and be value are subjective. But this at least answers your question.Why would anyone wants to replace this stock system?