Conservation of energy. It will take more power to move the water back up than you got when it moved down.
But how much is the waste? A 1% waste to turn a nuclear power plant which runs best at the same output 24 hours per day into a system which produces the most in late afternoons would be great. Not so good at a 50% waste.
No one is suggesting that it is free energy.
That said, during the day solar can provide power to pump it back into the reservoir and wind can do it at any time. If there is excess energy available, refill the reservoir as future energy for use when demand is high.