Because we want a 1920x1200 sized desktop. I've explained this many times on the forums so I'll try to condense the problem down.
Right now the display is 2880x1800 and it has an effective desktop size of 1440x900. Other 15" notebooks from every single manufacturer has been shipping a 1920x1200 screen size on their 15" notebooks for more than half a decade. Either as the default or a build-to-order option.
The rMBP supports scaled resolutions higher so that you can run the notebook with a desktop size of 1680x1050 or 1920x1200. Once PCI manufacturers abandoned 16:10 displays at around 2006 or so, they also dropped 1920x1200 as a common resolution, it's only really found mostly on 'workstation/business-class' laptops. Ironically I had a dell from 2003 that had this resolution on a 15 inch. But because the actual display size is 2880x1800 these resolutions do not look good. They look slightly smudged and when you move applications around the screen that use fine lines (text, 1 pixel high/wide lines) they begin flashing as the scaler tries to scale the pixel to fit.
See when you run 1440x900 on a 2880x1800 the scaling is perfect because there is 4 real pixels to represent 1 virtual pixel. A perfect square. But when you go out of this ratio either higher or lower than 1440x900 that's when you get the imperfect scaling and flashing of small details.
If the display was 4K then we could have a pixel perfect 1 virtual pixel to 4 real pixels with a desktop size of 1920x1200.
It's not that we want 4K so we "cant see the pixels". It's that we want 4K because that will make the display workable with a 1920x1200 desktop size. I hope this makes sense I know it's complicated to convey in comment form like this, it's much easier to show someone by using the actual laptop and changing resolutions so they can see the flickering for themselves.