I don't think that's the reason. Modern supercars have so much power that the average person that can afford them is going to wreck the drivetrain in a very short while if they have to manage all that power themselves. Automatic gearboxes are far more forgiving. You see the same with Porsches that have manual gearboxes, if you read out the ECU you'll see them overrev many times more than with the autos, if at all (in fact I don't recall seeing an auto that had overrevved).