The sharpness of HD or 4K video differs from real-life scenes because of the difference in resolution and pixel density. HD video has a resolution of 1920x1080 pixels, while 4K video has a resolution of 3840x2160 pixels, which means that 4K video contains four times the number of pixels as HD video. When we watch a scene in real life, our eyes see a much higher resolution and pixel density than what is displayed on a video screen. The higher the resolution and pixel density of a video, the closer it will appear to real-life scenes.
Another factor that affects the sharpness of video is the quality of the camera used to film the scene. In theory, it is possible for a high-end 1080p camera to film higher quality footage than a cheap 4K camera. However, most 4K cameras are significantly sharper than 1080p cameras. When footage is resized to a smaller size, the quality remains the same because shrinking footage works. However, a 4K resolution monitor would have a quality issue playing 1080p because it would be too small. Therefore, when we watch 4K videos on a 1080p monitor, we are seeing 1080p, which is 4K dithered down to 1080p. This is why 4K footage may look sharper on a 1080p monitor than native 1080p footage.