Welcome to the community!
Couple a reasons really.
For example the pmc.
They end up recieving your stock pcm core back and they proceed to flash it with upgrated most requested and sought features.
Then they resell it.
With the proper software, you can tune you'r stock pcm.
Same with stock blower,heads,transmission, and other things.
They end up modifing them to perf specs and they resell them.
Porly the most thought out reason for them is cost effective.
They don't have to go out and search for core's to work on.
