In real life they're mounted to vehicles.
AFAIK, you can thank the 1987 movie Predator for the idea that someone could walk around with a minigun as a personal weapon
Now I'm going to have to go watch that movie again. Not only was it so influential that it introduced the idea of miniguns as human-portable weapons to games, it's the source of this meme and what's not to love about a movie with Ventura, Arnold Schwarzenegger and Carl Weathers.