I haven't checked how this is presented in 5E, but I remember in 2E that the costs of the stronger healing spells that operated on more than hit points, and especially the Raise Dead and Resurrection spells had a very high cost in material components, and took their toll on the caster. In other words, not to be used lightly and all the time. Which means finding someone to cast it for you would come at a correspondingly high cost.

In a well-designed campaign world, that should be reflected in either a high monetary cost for the casting of such spells (a church requesting a sizable donation, for example) or some kind of demonstration that the target is worthy in the eyes of the church or its god.

This can actually turn into a storytelling and role-playing opportunity. Imagine you're blind, and you and your party need to prove that you're a worthy person while blind before they'll restore your sight. Or the whole party is made totally blind for the duration of a test or short quest that you have to complete together before the restoration spell can be cast.

All this sufficiently explains the existence of blind people. Lack of imagination is not an excuse for bigotry.

Also, a character may be unable to get their sight restored, and that can and should be explored for its role-playing potential.