What is the reason glasses are not sold for nearsightedness at places other than opticians?

Too many options: There are too many factors that determine a glasses prescription to allow for mass manufacturing. Generally you need a unique prescription for each eye that is combination of distance prescription, close prescription, location of the lenses on your face and any muscle imbalance correction. There is also tinting, anti-reflective coatings, anti-scratch coatings, polarization and more.