It seems to me that it's just the other side of the same coin, however. Pretty much all the other character classes come ready to play right out of the box. The thief needs stuff to steal, the fighter needs things to kill and weapons and armor to do it with, and the magic-user needs spells. All of these can be found in the box.
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Part of that is the modern perspective. We assume religion is a personal thing, a one-on-one relationship between worshiper and deity. This was not always the case. In the city-states of the Bronze Age, religion was a civic matter, and worship was a duty you owed your community, just like paying taxes and jury duty. It certainly wasn't a matter of personal choice, and thinking that you could have personal relationship with a god, like Odysseus or Perseus, was the height of hubris. Besides, most folks who ended up in personal relationships with the gods usually came to regret it.
The assumption in D&D tends to be more of the modern, personal relationship model that most of us who grew up in the West are familiar with. The cleric loses access to spells and powers if that individual deviates from the deity's dictates and interests, not if the god's favorite city gets sacked or the priests at the holiest of temples stop making sacrifices. The cleric is often expected to proselytize and bring more worshipers to their deity. Zeus didn't need folks wandering in barbarian lands spreading his word, and he didn't get his nose bent out of shape if you worshiped other gods, just so long as you kept the sacrifices and adulation coming. The Romans even made sacrifices to “the Unknown God” just to be sure they didn't offend some deity they'd never heard of due to ignorance.
But if you're expected to adhere to a set of divine strictures and spread the word of your god's greatness, you kinda need to know what those strictures are and what makes your god so great. The vague guidelines provided by the rules typically result in goody-two-shoes characters who make vague pronouncements on those rare occasions when they can address a crowd and must be conveniently facing the other way when the party decides they have to do something that the cleric's god disapproves of. (Or, even more bizarrely, tree-hugging, vegan druids who will starve the wolf's cubs by saving cute little lambs.)
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Art by Sir Lawrence Alma-Tadema and William Shakespeare Burton