Why do people have this misconception that dialect has something to do with intellect?

People whether we acknowledge it or not, automatically judge someone based on what we see, hear, and previous knowledge. Granted people with open-minds tend to ignore it, it still goes on. Personally, I don't use speech to ess intelligence. However, it does make a difference. For example, I have no problem swearing or using slang around my peers or friends, but wouldn't dare in a formal situation. Typically when I am speaking with a teacher, or when I'm applying for a job, I strive to make sure my english is imperial. That's just me. When I'm with friends however, I let it all hang out, lol.