https://en.wikipedia.org/wiki/Cornea
Incredible! Why are there any pain receptors at all?
Contact lenses are pretty much an impossible proposition. We can't get healthcare professionals to always correctly sanitize their hands, what chance do contact lens users (many of them teenagers) have of only handling their lenses and eye after sanitizing their hands, always sanitizing the lenses themselves in the proper, new solution and changing them on the right timescale?
And if you fail at any of these steps and get an infection, you are pretty much not going to come out of it without permanent damage of some sorts. You basically need to immediately get to an ophthalmologist and apply the proper eye drops, because even after a day you could have enough scarring to partially lose vision or increase sensitivity to light.
1: https://en.wikipedia.org/wiki/Immune_privilege#Immunological...
With leprosy, sufferers lack the ability to feel when they've injured themselves, they end up tearing the flesh off their limbs and breaking the feet down to a pulp through accidents as they can't tell they've injured themselves.
If you don't feel the piece of sand in your eye, then you're going to carry on damaging the eye until you do feel it. At which point, with minimal pain receptors the eye is likely damaged beyond recovery.
I had a scratched cornea recently, the scratch enabled an infection to take hold very rapidly.
So that you recognize, terminate, and learn to avoid situations that cause injury to the eye.