I wont say, that CAD is unusuable for 3d printing. We all use it. But as Olias says, it was designed / developed / invented many years earlier when nobody thought about 3d printing.
And then 3d printing was invented and to make it possible slicers where developed.
And CAD programms are still slowly adapted for the new needs. Maybe somewhere in the future CAD and slicers might become a union and everything works seemlessly.