Because it requires:
A) They have to have an "unmolested" transfercase/transmission in a vehicle
B) Taming the gorillas that typically tear these things out
C) Keeping that same gorilla from tossing that linkage and shifter into the scrap pile because he has done it like that 300,000 times before
D) Actually get all those little parts to land all in the same spot for you to pick it up, without those gorillas seeing it and thinking it belongs in that big scrap pile out back