I have noted this before (Destiny)

by cheapLEY , Friday, April 27, 2018, 11:14 (2479 days ago) @ Durandal

If the rumors of a late cycle reboot for D2 are true, I would guess just not having time to implement friendly AI was a big factor. Zavala is just an animation and occasionally popping a bubble on an infinite loop. I’d have loved to see even more scripted moments like that, but I still miss the friendly marines from Halo. The invulnerable Spartans from Reach weren’t quite as good, and AI Guardians would probably have to be closer to that than the fodder that Marines could be.