The process is different. Lenses can correct some types of poor vision and restore perfect sight, but if you have cataracts, for example, then improvement cannot restore it to 'normal'. Hearing aids are in essence, an amplifier and eq device. If your hearing is deficient by a small amount at certain frequencies so your hearing frequency response is perhaps a wavy line, then fitting an aid can produce a flatter line - BUT - with lowish fidelity. A decent pair of in ear monitors and a graphic produce better 'quality'. However, your brain can compensate for the uneven frequency response quite well, and maintain full fidelity. If your hearing is very poor at speech frequencies, then these can be raised by the hearing aid, even if the actual quality is not that high - so you can maintain conversations. Listening to music is pretty nasty sounding. I've often wondered how somebody with a hearing impairment would get on with decent in-ears and a decent multi band graphic or parametric? If you look at the frequencies selected for the hearing tests, they are not remotely 1/3rd octave! and they don't even bother much about accuracy up the top.