Peppers vary extremely from one pod to the next: adding capsaicin lets them closely regulate their heat quality control. I've had jala-yos* that I can eat on breakfast cereal (not that I have), and poblanos too hot to be used on an unsuspecting family for chiles rellenos.
Even when dealing with the vat-sized batches I'm sure the major manufacturers use, it's good to be able to balance their levels. For the same reasons, I'm sure they add glacial acetic acid (vinegar without water) and sugar to balance the tomatoes.
You'll say, "If the jalapenos aren't cutting it, drop a few habaneros in!"
But the flavors are different, and still hard to control.
* jala-yos are jalapenos without the pain