The argument could be made that they did cure Geordi La Forge's blindness using technology by means of his visor and later ocular implants. When I think about disability in fantasy settings I think of golem/automaton prosthetics replacing limbs and the like, and that most disabilities could be helped in one way or another. So to me the simple answer of why would there be disability in a world where magic should be able to cure everything is that not everyone has enough gold to pay for that magic or skill to wield it themselves. In that sense it feels very relatable to our real world, people unable to afford the technology and services that could help them