I don't think desktops require retina displays. There are an endless selection of big, bright, high refresh rate, high resolution, and cheap monitors to choose from.

Macbooks max out at 16 inches. Retina displays are (a good) attempt at trying to make laptops more viable for long use, but still they fall short of even the cheapest 4k decent nitts desktop monitor. They haven't adapted the tech, because it's not needed. The same pressures of selection aren't present.

The biggest font and text clarity nerd that I am aware of agrees on the matter, and makes some sensible suggestion and gives reasoning here: https://tonsky.me/blog/monitors/