For the same reason you turn to any other after-market modification. You're trying to make it better. We all know that things could be better, but for reasons of cost control, they don't always use the best parts. Why only electric fans on Lincoln Mark models, when the rest of us F-150's & Expeditions have clutch fans? Why don't Fords use a full synthetic factory fill? Why doesn't every car come with a K&N filter? Why don't we all have more aggressive and responsive tuning? Why those crappy manifolds instead of headers?