I've lost count of the times I've been told about traffic and asked if I'm visual with it yet
Enroute, perhaps 90% of contacts reported by a radar unit are never spotted - even if you have a plane full of pilots all looking for it.
This could be partly because I always fly as high as possible, and the majority of reported contacts are non-transponding and thus "level unknown" and they are usually flying very low. But for example I did a little local yesterday and got a contact reported at 5 miles. In the haze, one would not spot a 747 at 5 miles, and spotting a spamcan at 5 miles is very very hard at the best of times unless the contrast is excellent. In hazy wx, say 5k vis, most reported contacts will not be visible
within the criteria for ATC telling you about them.
There is some apparently very good new laser surgery which corrects both distance and near vision but a friend of mine researched it and found it is banned by the CAA.