The eye works in pixels, while the brain interprets in vectors. Literally, the retina has millions of cells, each of which send single-pixel signals back to the brain.
One thing some people don't realize, though, is that our pixel vision is much higher than the number of rods and cones our eyes have. Because we are constantly moving/shaking as we breathe and as our hearts beat, our vision from one moment to the next shift. And combined with the processing in the brain, this results in a much higher effective resolution than would be allowed by just the number of cells in our retina.
So while 1080p might look great, we're still nowhere near the actual biological limit. Just, we have diminishing returns compared to the cost.