There are 3 reasons.
Firstly, our sense of time. Animals communicate with each other about the present- about current moods and dangers, but not the future, whereas we can contemplate mortality.
Animals have various defences against predators (ie. their immediate danger) but we have just one defence against mortality - denial. No-one can deny we'll all die and rot, so some joker came up with the idea of an "immortal soul" which survives in the afterlife, and thus defends against death.
God's agents then only have to constantly remind followers of their mortality and convince them that the afterlife is managed by their god. The self-protective urges of the worshippers do the rest.
Secondly, holy men are aided by neoteny. It's a biological condition in certain species (yep, humans included) where juveniles act as adults, and adults as juveniles. Example: Humans start to reproduce when juvenile, and are filled with curiosity and playfulness. But adult humans are more like neotenous apes rather than adult apes- the apes lose their playfulness as they mature, but humans don't.
Still with me? Another example: Dogs. They're like neotenous wolves. Especially as we want our "best friend" to be playful, so we breed more and more juvenile dogs.
A mature domestic dog still leaps around and plays, like a wolf cub, but wolves mature and stop playing.
If your eyes haven't glazed over, the point is that a dog will respond to its human owner as if a parent. A dog can mate & breed but still obeys its master- it's a perfect pet.
Humans too can be sexually mature yet still need parenting. OK, you can see how this is fitting into place, right? Humans need some sort of super-parent, something as impressive to a human, as a human is to a dog.
So, we started inventing gods. Lots of gods. Female super-parents (Mother Goddesses), male super-parents (God The Father), or even whole families of gods. Like real parents, they protect, punish, and expect obedience.
But why can't real parents fill this role? Because they'd always need to be biologically bigger than their offspring, so we can look up to them, and they would need superior strength, to be biologically protective. That might be the case at first, but kids soon grow up and start breeding, whilst their feeble parents recede into the world of Richard Whiteley and the Daily Telegraph. Hence, we need all-powerful beings that we can (literally) look up to. No matter how old we get, we can still call our gods "Holy Mother" or "Father". Humans put child-like trust in them or their agents, who adopt similar titles themselves.
I'm Feelin' It!
The third reason that we lean towards such nonsense is our highly-evolved sense of cooperation. When our society became hunter-gatherers, we had to cooperate a lot more. As a pack of hunters, tribal leaders needed active cooperation rather than just passive submission. The drawback, then as now, is that any sense of initiative might lead to questioning our allegiance to the tribe or leader- we needed a fine blend of free-thinking and group cohesion. Of course, a super-being offers the chance for blind faith combined with thoughtful intelligence.
The problem is, some of us are still living according to our Neolithic instincts.
[Check out Pistonhips for more pics].
