That's because of the ascendancy of the prescriptivists of the previous centuries who were wrongly "guarding" "correct usage" based on stupid rules, mostly from applying the grammar of Latin -- a completely different language -- to English.
But in the modern era, in which a word search of most of English literature in the past several centuries is at everyone's fingertips, the descriptivists are winning. They simply have to do the word search to show that Shakespeare and the King James Bible, and Yeats, and Churchill, and journalists all used the the supposedly "wrong grammar." It turns out the "wrong grammar" was already common, and thus, by definition of what a living language is, the actual correct grammar.
In other words (pun intended), words like "ain't" are getting into dictionaries very quickly. All you need to do is read the yearly newspaper articles of "new words being added to the dictionary" to see how quickly neologisms and grammatical variants enter dictionaries and style guides.