Why do optical isomers always come in pairs?
Why do most people have LEFT as well as RIGHT hands?
The question is a bit of a non-sequitir. Optical isomers, the which are non-superposable mirror images, necessarily have left-handed and right-handed antipodes. Of course, only the one isomer may be encountered naturally. And it is a fact that most proteins and most sugars are handed. It is only in the past 20-30 or so years that chemists have developed the synthetic methodology to synthesize the isomer that is not encountered naturally.