I’m sure there’s a physical answer as to why a spherical lens can’t focus the entire image (or maybe it can, physics is not my area and I think it shows haha).
But the biological explanation is that your retina isn’t uniform, instead you have a very small region called fovea where the vast majority of your cone cells are concentrated. If you could take an instantaneous snapshot of your vision, like a picture, you’d be scared to see that everything is grossly blurry and distorted save for a very small circle where the image is very clear - that’s the fovea. Your brain takes multiple images where different parts of a scene are focused on the fovea to create a composite end result that looks better. The rest of your eye still captures light, but with less detail, and therefore it’s mostly dedicated to getting broad positioning of objects, noticing fast changes in movement, tracking peripheral motion, and so on - not on focusing on text or small details.
Curiously, you also have a circle that would be completely black if your brain didn’t fill it in - it’s the blind spot left by the insertion of the optical nerve, where your retina can’t capture anything.