Because they want to sell you a Retina(tm) monitor. I wish I was making a joke.
Roll back to a version of OSX that predates Retina, and _all_ of your monitors get the expected Mac-like font rendering, Retina or not. Go to 10.7 or newer, and all monitors are ran using the Retina tuning for font rendering, which makes it very smeary and blurry on normal monitors, but looks great on anything that triggers Retina rendering.
So, what I've been advising to the fewer and fewer Mac owners I know that want multi-monitor: only buy 4k monitors, OSX thinks they're HiDPI and won't fuck over your font rendering. At least, they won't today.
Roll back to a version of OSX that predates Retina, and _all_ of your monitors get the expected Mac-like font rendering, Retina or not. Go to 10.7 or newer, and all monitors are ran using the Retina tuning for font rendering, which makes it very smeary and blurry on normal monitors, but looks great on anything that triggers Retina rendering.
So, what I've been advising to the fewer and fewer Mac owners I know that want multi-monitor: only buy 4k monitors, OSX thinks they're HiDPI and won't fuck over your font rendering. At least, they won't today.