Why are PC screen resolutions so LOW?

Screen_resolution_medium

Take a look at that image right there, I'll explain it. The very bottom left features a box with a 4.5" diagonal measure and the aspect ratio of 1280 x 768, or, the screen of a Lumia 920. It has an orange line (inside of the blue line) that runs from the bottom left corner to the top right to mark it, and some orange text that reads 4.5".

That blue line runs from the bottom left of a dark gray/orange box to the top right, and that box, also measured diagonally, is 12.5".

Now if I lay out a grid of Lumia 920 4.5" screens end to end, it takes roughly 2.75 screens in either direction to completely cover that 12.5" area. As you can see there is some overhang, it doesn't take a full 3 screens to cover the full 12.5".

1280 x 768 in a 4.5" screen is possible, I look at it every day on my phone. If we simply multiply those dimensions by 2.75, we get a resolution of 3250 x 2112. Consider the MacBook Pro 13", which is as close as we can get to the proposed resolution. The rMBP 13" has a resolution of 2560 x 1600, which is 40.33% less pixels (4096000 vs. 6864000)!

Now I know that a lot goes into powering these screens, and I will fully admit to having zero knowledge of how computer hardware is created, how it is powered, how it is run, and the difficulties and challenges of writing software to scale appropriately to screens of varying resolutions. Those are all probably huge barriers to getting a screen with as high of a resolution as what I'm describing.

But is it theoretically possible? They can already cram the pixels into a 4.5" screen, and the battery on my 920 will last for 2 days straight. Not to mention the colors are gorgeous, the blacks are deep, and the 60hz refresh rate is stunning. Why not layout a few of these screen panels together and create an incredible PC screen?

Why is it that these Macbook retina display screens are considered as such an innovation when phones have been putting out screen resolutions that should blow those out of the water for a few years now?

If we doubled that, and made a desktop monitor of 15", it would have a theoretical resolution of 7040 x 4224, encapsulating 29,736,960 pixels. How fantastic would THAT look?