because they are works of art.
oh, the restriction is the turbine wheel
it would be very expencive to get each runner bent up perfectly to shape with no welding except for the turbo and head flange.
the dude that has the gibson r31 has priced up a replacment.... (same style as the fj one above) he told me $10,000 and had to be hydroformed.
the full race ones are expencive because of the materials they use and that the are welded by a robot. (very pricy tooling)