A fair question! Frankly, the main reason is a meta one. I don’t want to take class features away from the players. For a more Watsonian answer though, the gods are more distant in my games than I think is the typical assumption for D&D. A cleric certainly believes their powers are granted to them
by the gods, and with good reason. But there is also good reason to believe they are mistaken.
I think we do, yes. Or, rather, the metaphysics of my games intentionally diverge from the metaphysics of typical D&D. Like I said, the gods are more distant. Clerics entreat the gods through ritual and prayer, and can produce miraculous effects through them, but there is room for doubt whether the magic is truly the result of divine intervention, or of the ritual itself. After all, Warlocks summon powerful planar entities and bind or negotiate with them to convince or force them to do magic on their behalf, who’s to say Clerics aren’t unknowingly doing the same thing?