Because they aren't separate.

Written speech is an emulation of spoken speech. This much is true and has always been true. However, the advent of mobile text-based instantaneous communication has caused spoken speech to be influenced by written speech. Tell me you've never once in your life said "lol" out loud. That right there is a keyboard shortcut of a textual maladaptation turned into vernacular and recycled back into speech. Same with the verbal use of "hashtag." It is a verbal representation of a textual shorthand used to express an idea larger than its ideogrammatic payload.

There is no easy, immediate way to communicate the verbal "the Us(ual)". So while it may be used amongst an ingroup as a part of their spoken language, it's a tic - it's "an idiosyncratic and habitual feature" not an idea that can be communicated to others.

Verbal languages are effectively dead now - if you don't have a written language to go along with your verbal language, you have no ability to transcend your immediate environs. "How do I write this slang" is a de facto and de jure attempt to transcend the immediate environs of language and it is a failure. That it can easily be communicated verbally doesn't matter - Kalahari bushmen can absolutely use language features no other language does but they don't catch on with anyone who doesn't hear them spoken.