Why? Because it is not a good use of the R&D and production money. You can get much more out of the same cost if your car is designed with better engine (i.e. variable valve timing or variable intake opening) than different tubing length / width.
Also a much bigger header only increase top end HP at the expense of bottom / mid range, so why bother?
Mazda use it for their skyactiv motor because of their piston and injector design that can benefit more from it. It is not as useful for the more common / old school engine design.
"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah