I think it's because its safer that way. These days a Captain does not supervise a co-pilot, both pilots supervise each other.
Modern helicopters are also so complicated that without a Type Rating (which can take a month of full time study and simulator flight) a pilot would struggle to make a valid contribution. Having said that, Type Rating training is still done on aircraft under the supervision of a TRI or Check Airman so this is an example of co-pilots gainig experience (training) on type before sitting a licensing exam.
It was not always like this with pilots and at one time an IR was only required by the Captain with the co-pilot flying (and learning) IFR under their supervision.
But the real reason is that flying requires much greater skill than spannering and you should never waste any opportunity to tell your engineers that if they had tried harder at skool they could have been a pilot like you
