Sizing to pixels works great when the device manufacturers all use approximately the same ratio of resolution to viewing distance. The problem with that approach is that it doesn't allow for nicer, high-resolution displays.

Sizing to pixels is vital when using low-resolution displays, since any scaling will be unacceptably blurry.

Apple's approach with their Retina displays was brilliant - make the high-res screens an integer multiple of the low-res ones. That made the scaling issues much easier to deal with.

The tragedy is with Windows. Microsoft has long emphasized font hinting, which results in sharper display fonts. But it means that the font metrics don't scale linearly with the resolution. Apps that weren't tested at anything but the default resolution would invariably have rendering issues, so nobody used oddball resolutions. Because nobody used high resolutions, the app makers didn't feel the need to test them. Chicken/egg problem. It has taken far too long to overcome this.