You should also note that this phenomenon isn't just restricted to the movies. Like already said, it lends a certain realness to the characters in a movie, but this is just because we know it to be real. It isn't just made up - it's as real as it gets. You hear people talking like that anywhere you go everyday. Depending on their origin and social standing people are more or less likely to 'abuse' the grammar of their native language in some way.
The examples david11 brought to the table aren't really crass. In fact, they can be encountered across all American and even British social "classes". This is how language evolves. Language isn't just one fixed concept - it's subject to changes at any point in time, ever changing so slightly. Some say it's a bad thing, others however, do embrace this phenomenon, because they see languages as what they are, living constructs.
Those changes happen and will continue to and there is nothing that anyone could do to prevent this. The main reason for those changes is pragmatism. If it's easier to express something in a certain way, people will use that method. Or, people who are not so sophisticated in their use of their native language actually make mistakes and spread those around. Others may very well know that those usages are wrong, but they could also see something in there that offers a certain "advantage" in their eyes, and so they pick it up and spread it even further. Like, for example, in "Where're my shoes?" vs "Where's my shoes?"
If enough people start using it, it's considered to be part of the (spoken) language. The written language is much more resilient against changes, but it evolves too, only slower.
No native speaker is using their language 100 percent correctly or to its fullest potential, especially in hasty times like these. Some can't speak properly, because they just don't know better, some could but don't want to, and others actually correct their mistakes as they make them.
This is one thing every language learner should keep in mind: you don't have to speak the language you are learning perfectly, because those who speak that particular language as their native language, make lots of mistakes themselves. You just have to make the same mistakes