Because it helps them sleep at night.

Only conventional literary comparisons are acceptable, and all creative or unorthodox structures, regardless of their grammatical correctness, are to be shunned because they're indicative of someone's being a retard. God forbid I use other ways to convey imagery and allusions to help relate a thesis.

It's the same for memes - esoteric pop culture jokes like "Where's the beef?" that predate the current generation's birth are timeless classics, but esoteric pop culture jokes referencing current celebrities get old and tiresome.

...of course, the counterpoint is that the word "literally" is used itself to 'defuse' literary structures and indicate that they are not, in fact, hyperbole or metaphor but an explicit relation of factual experiences. Using "literally" to modify a metaphor is somewhat redundant in this context.

But on the other other hand, it can further enhance hyperbole when appended on an outlandish metaphor to suggest the absurdity of the metaphor when taken concretely.

So really, it all goes back to how the speaker wishes to convey a point to the listener and their personal style.