People are taught how to write in an academic style in school.

There's different applications for writing. Some examples in include but are not limited to academic, creative, and conversational/casual writing. Education tends to focus on academic(correct grammar, writing essays, doing research papers) followed by creative writing which is storytelling and poetry.

Expecting people to write academically in casual settings is weird. If I write a letter or email to my friend it's going to include slang and have looser grammar.

Casual and academic writing have their place and time. Suggesting casual writing is inferior is just silly. If people always wrote everything like some formal essay it'd be fucking boring. No humor, no colorful language. Just boring ass academic writing everywhere.

Plus, believe it or not some of my most brilliant professors had a very casual way of speaking and writing. Academic use of language does not indicate intelligence. It just means they know how to write in an academic setting. Nothing more, nothing less.