If the locked ones did not show up people would purposefully not unlock new units so they would have less options in the chest and have a higher chance of getting what they want.
It wouldn't be a Supercell game without that sweet old RNG But yeah, about the question of why this is in the game. In the old version who could PAY to get more dice rolls. If that feature is not in the game yet, they will for sure add it later. It's just to make people want to buy dice rolls, characters and extra keys
