GNOME dev team (and by extension GTK) probably don't care that much, iirc most of them use expensive macbooks so many issues get ignored because it "Works on my machine" (e.g. several issues with font rendering that don't affect retina displays)
The font rendering thing is more complicated than that. Subpixel antialiasing doesn't work with animations and I'm pretty sure it interacts poorly with fractional scaling too. The fact that you don't need subpixel antialiasing if you have a HiDPI display just lets them make the tradeoff be "you need a good monitor" instead of having to choose between jaggy text or weird shimmering on animations.
Also, it is impossible to know which subpixel structure the user has.