They can influence people in ways you really wouldn't expect. For example, people in countries where exposure to movies and film is ubiquitous behave differently, in general, to being shot. It appears that seeing people get shot and falling or being flung down by the impact of a bullet becomes internalized, adding a psychological element to the trauma of gunshot wounds (GSWs). Meanwhile, as NATO and in particular US forces noted in Somalia (as well as elsewhere), in countries where there exists far less exposure to action flicks, cop shows, and other TV/Films depicting people being shot, soldiers would hit a target causing lethal trauma, but instead of falling down or freezing up, the target would simply move to new cover, return fire, then suddenly die almost as if they never noticed they were shot. Even something so physical as lethal trauma has a very psychological component that media exposure can influence.
Also, consider why Modern Standard Arabic isn't the only "standard" Arabic dialect, as informally Egyptian Arabic is spoken just about everywhere Arabic is a or the native language: Egypt is basically the only place where movies in Arabic are made (and, since Modern Standard Arabic is an "artificial language" but is also the dialect used for newspapers and TV broadcasts, exposure to native Arabic dialects is further limited).
That said:
1) People influence media content. The reason that movies like the Saw films or series filled with graphic sex and violence like True Blood or Game of Thrones exist is because they have an audience. Restrict televised media to children's shows, lectures on academic subjects, and g-rated movies and you'll find that the internet will fill the gap before you can blink (indeed, it has already started to challenge mainstream TV/movies with e.g., miniseries on YouTube, straight-to-Netflix productions, etc.).
2) There are limits on what can and can't be (or at least can't easily be) internalized or influence action and thought. To see this, consider watching videos that are supposed to teach you how to shoot, to learn Karate or Krav Maga or some other martial arts, to play piano, to dance, or even to learn pre-calculus or calculus. To the extent such things can be internalized, it is only via practice. You can watch someone solve differential equations all day or spend a week watching some series "Learn Muay Thai in 7 days!" or something, and get basically nothing out of it because this kind of passive learning limits what can be learned. Playing violent video games, watching graphic movies, etc., is such a weak predictor of behavior as to be non-existence, especially when you factor in the direction of relationship (those who act violently and love violent media are likely drawn to violent media because they like violence rather than liking violence because they watch violent media).
3) There is a difference between being responsible for one's actions and being influenced. It is impossible to not be influenced by a myriad of factors, from family and culture to epigenetic developments in the womb. Everybody is influenced by their surroundings and upbringing. Remove the typical influences (such as more than absolute minimal interactions with other humans) and you get feral children who will likely never be capable of speech, social interactions, or most of the things we as humans do. We're social animals. Being influenced by society and its "products", though, doesn't negate responsibility.