Tbf even the gau-8 used by the A-10 warthog only has 18 seconds of brrrrrrt. So if anything carryable could last 5 seconds, you'd basically be out of ammo anyways. It's a way of making it "realistic" without limiting you to the point of making it worthless. Which in reality, in any extended engagement, a ground mini-gun is absolutely worthless. No one person by themselves could use that gun effectively against more than ~10-15 people and only in VERY specific situations. Only in a kamikaze rush would it be of any value. I'd take 3 marksmen with bolt action rifles and iron sights over one dude on foot with a mini-gun all day, every day.
Mini-guns are very, very good in specific applications, but absolutely worthless in others. But it's fun in a video game to weird. It'd be unrealistic and OP to carry with enough ammo to make it useful outside of fallout power armor, or a full mech.