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Your eyes have a central area of the retina called the fovea which is more densely packed and has much higher acuity than the rest of your retina. That’s the area you use to pay close attention such as for reading or hunting for food. The rest of the wide field of view is more useful for detecting movement than anything else.
It’s not a flaw—it’s just the best solution for the problem of survival.
It's a bit of optics, a bit of eye physiology, and a bit of how the brain works.
Optics: Your lens focuses light on a focal point. For a sharp image, this point in space should be on the retina surface. Both lens and retina are not ideal geometric objects, so directions and angles can matter.
Eye: As others said, the retina has different regions, with different amounts and types of photoreceptors. Some regions are good for a high resolution image of whatever is in focus, others are mostly used for peripheral vision.
Brain: Your brain still gets all the data, from in and out of focus photoreceptors.
Maybe it would be possible in terms of optics to focus on more than one thing at the same time. But retina composition and brain architecture are adapted to the optics which we have: One way to focus, and peripheral vision around it.
With a bit of training it should be possible to mentally focus on image parts out of physical focus; mentally focus on something in your peripheral vision. You would mentally concentrate on a physically low resolution image (lower receptor density), it might be distorted (lens optics), and your brain might not be used to use data from these receptors for this task. So the result probably still feels like "I can't focus on that".
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.
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
This is what interests and distresses me about the mysteries of the human body. I once saw a video about capturing a person's recollection of his memory of a video clip by scanning his brain activity. The result was really obscured and blurry, but it actually did resemble the clip, which was deeply disturbing. I would have never known my actual vision is vastly different from what my brain makes me perceive to be.
What you perceive is vastly different than a continuous filmstrip. Have you ever tried to watch unstabilized footage from a person jogging? Totally unwatchable! But your brain smooths it out better than the best steadicam. Tilt your head from side-to-side: what you perceive stays upright. And of course you know hat your eyes don’t smoothly pan from subject to subject but are constantly “saccading” around, but your brain processes that all away.
Visual processing is amazingly complex. Its also interesting that our other senses have different levels of processing. Our sense of smell is nothing compared to a dog’s, our sense of hearing is nothing compared to a whale’s, etc. A metaphor I heard once (don’t know how accurate it is) is that when a human walks into a kitchen they might smell that a stew is cooking. A dog would smell both the overall smell but would also smell the individual carrots, peas, chunks of meat, etc.
I think a useful way to think about it is that your perceptual brain isn't in the business of making you see, or hear, or anything like that - it's in the business of giving you an accurate-enough-to-be-useful idea of what's around you. You see the world as being sharp and stable and consistent even though the literal visual signals going from your eyes to your brain aren't... because the world is sharp and stable and consistent. Or at least it's enough those things that it was useful for the brain to evolve to generate that specific perceptual experience. The signals coming from your eye are just (some of) the information the brain uses to generate that experience.
A) Brain. You can train yourself to use more of your whole field, but you'll lose that attentional "spotlight" that you normally aim at things and picks out all the detail.
If you're ever scanning something like a thick tree, looking for something, hold your arms out and form your fingers into a square frame. Use this to further focus your attention, and scan the tree in a more methodical way by moving the finger frame. You are more likely to find the thing you are looking for this way.