Why don't native English speakers from the U.S. always use proper English?

I don't know anyone from the UK and about their English when they're not writing it on the internet but why do native English speakers not always use proper English and someone who's a native German speaker, French speaker, Japanese speaker, or Spanish speaker etc. use proper English more often than us native Americans? Example of terrible English: "I have spoke with the professor and he said the reason why Raymond was getting terrible grades is because he was missing a lot of assignments." I mean just one word in there is wrong but there are also a lot of instances when people say "It don't matter" as an answer to a question. Now that grammar is totally incorrect than the one I have above.