Lifting something like that for a minute, and carrying it for days AND keeping it in control while firing are completely different things. And I know the movie, one of my favorites. But again, its a movie where they just use things cuz they look cool and usually get things wrong or make them seem easier than they really are. They most likely carried one made of plastic. Also, look at the guy that carried it, pretty big guy ( Jessie "The Body" Ventura, ex-pro wrestler and body builder... soo, big guy). Think the small guy in glasses would've managed if it was real? Also, thats just the weight of the gun itself, keep in mind, the guy needs to carry ammo for it as well, which would weigh a lot more than that. Besides all that, theyre just not practical. Blow your load in no time flat, its essentially spray and pray, cant aim until you start firing, guy firing it is exposed making him a target. And if you watch the real sequel, Predators, the Russian guy carries one too, and just after they meet him in the beginning, he complains about the damn thing being heavy, and hes no small fry either.
The main thing im getting at is that its big and heavy, and our agents have a lot of other gear already, and it wouldnt work with the ammo pack version since we already have backpacks with our weapons and skills attached to em.
Edit:
Just got a little curious and looked it up, while the minigun IS real and still in use, the handheld version is pure fiction... like i said above, just cuz it looks cool and movies get things wrong because of it. And for the same reasons why the minigun is impractical, the microgun project was abandoned.