I asked this very question whilst attending a TSLG meeting some years ago and was advised that the background to the IMC rating for aeroplanes goes back to the issue of plank drivers getting airborne and then 'encountering unexpected' bad weather that forces them to have to fly IFR to get safely back onto some tarmac.
Better to equip them with the 'skills' needed to save the day rather than they crash and burn.
Helicopters of course can slow down, go down and if necessary land in fields so there was never an equivalent safety case for an IMC(H).
The fact that some IMC(A) holders treat it as a 'cheap and cheerful' IR and leap off in obvious 'dodgy' weather is another matter.