You had to understand that these two left eyes were incredibly unique. They came into existence when Song Shuhang was struck by divine punishment, reduced to nothing but a head, and then, during the mysterious Holy Ape Dragon Power Technique cultivation, a crafty fairy plucked them out to serve as spare eyes.
These two left eyes were infused with potent powers of rebirth and the essence of life, starkly distinct from Song Shuhang's normal eyes.
To obtain eyes with the same unique abilities, one would have to invoke divine punishment again and reduce Song Shuhang to a similar state, and then repeat the entire process to extract similarly special glass eyeballs!
Any missing step in this process could lead to errors.