The 2004 30" Apple Cinema Display was 98.44 PPI and cost $3300 at launch. Today that same $3300 buys you a 31.5" 4k monitor at 140ppi. I wouldn't call that "pretty much the same resolution".
Also it's coming to devices which don't have to rely on 3D performance. It is an astronomical hit rendering on such high resolution, because you still need anti-aliasing.
I figured out this Dell screen is 235 ppi so pretty similar to Macbook and other 'retina' equivalent laptop displays.
And I was comparing the current 27" 2560x1440 Cinema Display to the 30" 2560x1600 Cinema Display. The ppi increased a bit, but it is essentially "pretty much the same resolution".
The fact of the matter is that manufacturing large, high-resolution displays is expensive, and that's due to a number of factors: the limited market for high-end desktop displays, the 2-4x increase in subpixel failures as resolutions scale 2-4x, and Windows.
Now that 4k tech is figured out, and Windows is no longer a complete failure at scaling to high pixel densities, expect the market to increase and prices to fall accordingly.