I have to design induction systems as part of my trade.
There are three main reasons why all cars are not fitted with such induction systems:
1. Noise - 90% of owners would find the noise level of an open intake far too loud and intrusive - it's only Petrol Heads who actually like the extra noise. Us engineers have to go to extraordinary lengths to reduce the noise without harming performance - in some cases we can improve the torque if we get it right.
2. Maintenance - dealers can't be doing with washing & oiling of foam / cotton filters - it takes too long
3. Air intake temperature - in hot climates, the temperature of the intake air is critcal for performance - too warm and you lose performance big time (you get a double whammy - lower density and more knocking) Most induction kits pull their air from the general engine bay which can be 20C hotter than the outside air.
Hope this helps
