If you have more than one display, and you want to be able to drag windows from one to another and want them to have a similar size after dragging, it's a hard problem.

You have never used two monitors with different scaling (eg laptop and desk monitor), and have never moved windows between them? Ever?

If so, good for you, but there are plenty of people who do so.

Hell, in X11 it even sucks if you use the screens one at a time, eg if you turn plug your laptop into the monitor and turn off the laptop screen, and then later unplug and continue your work on the laptop, the scaling will be off on at least one device.