I don’t believe you’ll see a 12v impactor - or even hydraulic impactor because of the ammount of energy lose and the reaction force though the tool.
in other words - a good bit of rotational energy is lost to the impact mechanism - they need power. Meanwhile though when impactor is swinging there’s a significant reaction in the handle.
18V systems with their bigger batteries help to counter this - making them easier to use. Buy the time you do all that to develop a 12V impact driver - you’d get just as much torque out of a quality motor for less.
which is why you do see 12v drivers and drills.