Your Eyes See Barely Anything At Any One Time
Your eyes feel about as big as they need to be, letting you see mountains, oceans, or your entire mom all at once. Except you're not really seeing much of any of those. The light-sensitive cells in your eyes are mostly concentrated in a section called the fovea, and as we've mentioned before, the fovea is super tiny. Imagine a one-degree-wide cone opening outward from your eye. Everything you see in detail fits in that narrow cone. As proof, keep your eyes totally still and try reading this entire page. You can't. Everything beyond "You can't" is just a blur.
It's just as well that most of your light-sensitive cells (called "cones," not to be confused with the imaginary vision cone from the last paragraph) are all dedicated to the exact thing you're concentrating on, because you don't have many to spare. You have about 6 million cones total, and since each is sensitive to just one primary color, that's the equivalent of about 2 million pixels, or the same as a 1080p image.
A 4K image, by comparison, has 8 million pixels. "Wait," you ask, "then how can I even see 4K?" Answer: You can't. A 4K movie looks good because the tiny section of it you're focusing on at any moment looks awesome, and you don't see the rest in detail. If every bit of the image other than those few centimeters were at a much lower resolution, you wouldn't even notice.