Why do the Outer Gods and Great Old Ones have cults?

(This article is OOC and will be primarily for DMs)

Throughout the history of Dungeons and Dragons there is an influence of the Lovecraftian Mythos. It's common knowledge that a lot of Gary Gygax's original design philosophy for the game was based on the Conan series by Robert E. Howard, who was close to Lovecraft, and whose work would show influences from cosmic horror tropes. In the First and Second Edition Deities and Demigods Cthulhu is specifically named as a powerful and influential entity in the world of the game, yet subsequent editions have never been exactly sure in what way to utilize these great old ones in a way that satisfies a majority of the audience. In 5e there has been a new influx of mechanics that support the Great Old Ones in the form of warlock patrons and a few select monsters but there is not enough fluff to support these characters and most frustrating of all is the lack of motivation and context (some might argue that this isn't necessary, but in my opinion it is fully necessary in order to create a cohesive and interesting narrative).

One of the largest problems in my mind for bringing the Lovecraftian Mythos into the game of Dungeons and Dragons is that many of the stories that don't involve the Elder Gods focus on how the existence of the Great Old Ones and Outer Gods is traumatizing, how one could worship a God for all their lives and have no proof of their existence, yet these beings could be seen plainly as being real (while being entities of malignant evil). This is part of what supports the cults that form around these deities in the ordinary mythos, as it makes sense to worship the real, proven god over one that could be false. This is much more difficult to justify in the Dungeons and Dragons world where the gods are more hands-on and involved in the prime material plane, but there are still tropes that can work and justify the support and worship of these cosmic threats.

The way to make the Great Old Ones seem like a serious threat is to borrow from the tropes of the original stories, but to twist them ever so slightly. One needs to think about how and why the Outer Gods were prevented from accessing the planes of the Great Wheel and also why the Great Old Ones were sealed away by multiple Elder Gods instead of being outright slain. Events like the Dawn War saw the deaths of Gods and Primordials, yet when the Gods saw these Eldritch horrors they were imprisoned instead. This both adds tension and weight to the power of the Great Old Ones and the possibility of their escape as well as providing a simple singular motive for most of the Great Old Ones: escape. Beyond that the only motivation necessary is corruption of all things. The level of devastation and the threat of the world being rebuilt in a terrible image is the very thing that could motivate cultists to worship them. The promise of a new start or an end to a world they despise can be a powerful force to inspire the hopeless as can the promise of their incredible power and the relative scope of their power to that of a player (particularly at low levels) makes the relationship of the warlock and their patron make more sense, as well as adds some potential horror tropes.

In the world of D&D the worship of beings can increase their power, and through rituals and sacrifices parts of the world can be devoted to a deity, expanding their influence. If the current purpose of each Great Old One is to escape their cosmic prisons this is an easy way to achieve their goal and with each incremental increase in their power outside the prison, the amount of power they are able to demonstrate is able to increase in turn. The actual act of freeing them is no easy thing, however, and should always seem an insurmountable task until a great amount of strength has been lent to them. In the original stories their prisons are placed in places where mortals dare not or could not possibly tread, so consider the locations for their prisons. Under the sea in deep abyssal caverns, inside of volcanoes, trapped in pocket dimensions, sealed inside of a star, etc..

Another possibility is that the cults are created separate of the Great Old One to which they are dedicated. These beings of tremendous power might not need the support of mortals to escape their prisons or expand their influence. This does not mean that any cult that forms would be of no use to the deity, however. When worship provides power and increases influence this is always a benefit, however the promises of the Great Old Ones can be manipulative if not just outright lies. The cults of Cthulhu were promised eternal life as long as the stars themselves, but this failed to specify their quality of life or the forms in which they would live.