Mustangs! They're cool, they're fast (well, most of the time), they're stylish, and they represent the spirit of American driving arguably better than any other car. That's why they're in movies. Lots and lots of movies.
This infographic from Late Model Restoration shows just a handful of the more famous Mustangs to appear in film. There's Bullitt, obviously, both "Eleanors" from the Gone In 60 Seconds films, the GT500 from that awesome intro scene in I Am Legend (wouldn't you want to hoon that car around an abandoned New York City?) and many others.
I'll also say this: while we aren't sure exactly what the next Mustang will look like, I'm sure it will be a damn sight better than the 2015 Mustang that showed up in Back To The Future II. It probably won't fly, though.