Because when shooting, you focus your vision on a pistol iron sights, so you need to see perfectly sharp on a distance less than 1m. Not sure if you can “cheat” with glasses to have better vision on that distance, if your vision is healthy
Once you raise your arm for real, you don't really need to look at the target, your hand and hence pistol will be in the right spot.
All you need then is to fine-adjust the sight alignment, hence why you focus entirely on the front sight post.
Once you get good you just know how to stand and such, so you'll be mostly bang on right away.
For this reason, if you get glasses for shooting, you'll want them made so your best focus is at the front sight.