I would not say that fear of death can be identified as the primary motive for religions, though it has surely played a part.
My reason is simply anthropological. Unless you define "religion" as tautologically assuming an "afterlife," there are religions, or systems of divine entities, entirely lacking in the rewards of heaven. The Greeks were by no means unanimous on this, and Buddhism, while not a monotheism, seeks absence of afterlife through the guidance of divine entities. In Egyptian religions, the concept of "afterlife" apparently underwent a kind of democratizing process, applying at first only to the pharaoh and then eventually to everyone.
Arguably a more universal source of religion is the very impulse you cite, the impulse to explain and predict. The Gods are causal forces that have not only "efficient causality" but "purposes" or final and teleological causality. Which makes sense. How else would we be able to investigate why and how these intangible forces go about things? We may find between their "reasons" and our own some similarity or correspondence to be worked out.
Such enticing correspondence between the way the universe works and the way we think and reason is also the gateway into science and survives vaguely in such ideas as Wigner's "unreasonable effectiveness of mathematics" or the "anthropic principle" or even Einstein's "god does not play dice with the universe." These, of course, hold no brief for some Levitical patriarch in the clouds. Yet they note the dialectical, reflective, or discursive nature of our being in the universe.
Death is certainly one of the questions that arises. The individual appears to utterly cease, yet we are also aware that the mortal individual transmits the "immortal" regeneration of the species...as well as the "other species" we consume.The finite lifespan is clearly transcended in some sense, and individuals are also "remembered." But if we are an "effect" of species being, what "causes" the species? What is its purpose? How else could we ask that except in some sort of "dialogue" with the universe? And how could we picture this as a dialogue if we heard only random static on the phone line?
In short, to take a purely physicalist stance and see the whole of religious narrative as a psychological distortion "caused" by fear of death is crude, preposterous, and evidentially dubious.