You misunderstand - the MacOS software stack can only scale the UI to 1x,2x, 3x. So you can either use 4K display at 2X scaling, which gives you fullhd worth of realestate with Retina resolution. Or you can use it at 1x, ehich gives you tiny icons.

When you use 'looks like 2560/qHD', then the OS renders to a virtual surface, pretending it has a 5K screen attached. Then it downscales that image, and outputs it. The result is janky straign lines, blurry'er text, etc.

Modern Windows and Android render their UI natively in any resolution, and so you don't get issues. The caviat is that windows has 3 UI frameworks, and the oldest one is still found is some places, that's the one that doesn't scale.