Why don’t the orcs have gods? Do orcs not generate mana, or can they not direct it to a deity, or what? Is “god” something a being can become if they are worshiped enough? Was Orcus a proper god, gaining power from his worshipers? Could he not do so because he wasn’t a god? In either case, why didn’t another god or pantheon claim the orcs as worshipers after his death? Is the power of a god is reflected by, or derived from, worshipers? If power is derived from worshipers, it seems like they were leaving power on the table by not recruiting them.
They do have some gods, however, by and large they are shamanistic rather than pantheistic.
The shamans are their priests. Check out the Library on Shamans/Druids or Druid/Shamans and how that works. T-A-G probably needs to update the shaman stuff a bit more. But the shamans can certainly harness the mana of the tribe etc to do magic in battle and elsewhere this will be seen in book 3.
So, by and large, think of them as being more like Native Americans, or various pre-European African tribes.
However, they do/did refer to Orcus as a god. Tom is learning much more about this in book 3. There are a few other war gods who tend to be quite somewhat bloody and require sacrifices of the tribes enemies.
And what Orcus was actually up to is a major learning curve for Tom and the readers, not sure how long until that is more fully revealed. But a big chunk is revealed in Book 3.