As Carmody notes, they have good damage codes and are the best weapons at Near (medium) ranges.
A more interesting question is "why would anyone take an SMG instead of an assault rifle?" Right now there is no mechanical reason to take an SMG, given than they have a lower damage code than assault rifles and can't shoot Far targets.
To balance this, our group gives assault rifles a -2 modifier at Close ranges (instead of OK) to represent the challenges of using a long barrel in tight quarters. That gives SMGs an advantage for up-close work.
Those are considerations from the player's perspective. As a GM, I give the NPCs whatever it makes more sense for them to have, even if it's not mechanically optimal. Sometimes an Uzi makes more sense than an AK, and sometimes an AK makes more sense than a shotgun, even if autofire isn't a thing, rules-wise.